Introduce `flex-list` & `flex-item` elements for Gitea UI (#25790)

This PR introduces a new UI element type for Gitea called `flex-item`.
It consists of a horizontal card with a leading, main and trailing part:


![grafik](https://github.com/go-gitea/gitea/assets/47871822/395dd3f3-3906-4481-8f65-be6ac0acbe03)

The idea behind it is that in Gitea UI, we have many cases where we use
this kind of layout, but it is achieved in many different ways:
  - grid layout
  - `.ui.list` with additional hacky flexbox
- `.ui.key.list` - looks to me like a style set originally created for
ssh/gpg key list, was used in many other places
  - `.issue.list` - created for issue cards, used in many other places
  - ...

This new style is based on `.issue.list`, specifically the refactoring
of it done in #25750.

In this PR, the new element is introduced and lots of templates are
being refactored to use that style. This allows to remove a lot of
page-specific css, makes many of the elements responsive or simply
provides a cleaner/better-looking way to present information.

A devtest section with the new style is also available.

<details>
<summary>Screenshots (left: before, right: after)</summary>

![Bildschirmfoto vom 2023-07-09
21-01-21](https://github.com/go-gitea/gitea/assets/47871822/545b7da5-b300-475f-bd6d-b7d836950bb5)
![Bildschirmfoto vom 2023-07-09
21-01-56](https://github.com/go-gitea/gitea/assets/47871822/b6f70415-6795-4f71-a5ea-117d56107ea1)
![Bildschirmfoto vom 2023-07-09
21-02-45](https://github.com/go-gitea/gitea/assets/47871822/47407121-3f2a-4778-8f6d-ad2687c2e7b3)
![Bildschirmfoto vom 2023-07-09
21-03-44](https://github.com/go-gitea/gitea/assets/47871822/76167aaf-c3b2-46f6-9ffd-709f20aa6a34)
![Bildschirmfoto vom 2023-07-09
21-04-52](https://github.com/go-gitea/gitea/assets/47871822/af8fdde5-711e-4524-99cf-fb5d68af85b9)
![Bildschirmfoto vom 2023-07-09
21-05-25](https://github.com/go-gitea/gitea/assets/47871822/ae406946-e3e4-4109-abfe-b3588a07b468)
![Bildschirmfoto vom 2023-07-09
21-06-35](https://github.com/go-gitea/gitea/assets/47871822/2dbacc04-24d6-4f91-9e42-e16d6e4b5f1f)
![Bildschirmfoto vom 2023-07-09
21-09-03](https://github.com/go-gitea/gitea/assets/47871822/d3ca4e56-a72f-4179-adc8-98bfd638025b)
![Bildschirmfoto vom 2023-07-09
21-09-44](https://github.com/go-gitea/gitea/assets/47871822/df1fa689-499c-4e54-b6fb-3b81644b725f)
![Bildschirmfoto vom 2023-07-09
21-10-27](https://github.com/go-gitea/gitea/assets/47871822/b21cac71-a85a-4c8c-bb99-ab90373d8e09)
![Bildschirmfoto vom 2023-07-09
21-11-12](https://github.com/go-gitea/gitea/assets/47871822/89be39cf-0af9-4f2d-9fca-42f9eb5e7824)
![Bildschirmfoto vom 2023-07-09
21-12-01](https://github.com/go-gitea/gitea/assets/47871822/079579ea-1ecb-49c0-b32b-b59ed957caec)
![Bildschirmfoto vom 2023-07-09
21-17-44](https://github.com/go-gitea/gitea/assets/47871822/61ac6ec4-a319-4d5c-9c99-2e02a77295ba)
![Bildschirmfoto vom 2023-07-09
21-18-27](https://github.com/go-gitea/gitea/assets/47871822/5b55b73f-6244-47f7-a3e6-c5e4a7474585)
![Bildschirmfoto vom 2023-07-09
21-19-18](https://github.com/go-gitea/gitea/assets/47871822/c1b7c22e-3e5a-46d4-b8d6-5560db478c0b)
![Bildschirmfoto vom 2023-07-09
21-29-13](https://github.com/go-gitea/gitea/assets/47871822/82ffca8d-ab2e-4a18-9954-5b685bf6a422)
![Bildschirmfoto vom 2023-07-09
21-30-11](https://github.com/go-gitea/gitea/assets/47871822/ad2fdccc-2be8-41bb-bfdc-a084aa387b61)
![Bildschirmfoto vom 2023-07-09
21-32-44](https://github.com/go-gitea/gitea/assets/47871822/2d298ba7-d084-48b5-a139-f86d56262110)
![Bildschirmfoto vom 2023-07-09
21-33-28](https://github.com/go-gitea/gitea/assets/47871822/4cbd838e-9de8-4ad0-8ed9-438da5c9a5cb)


</details>

---------

Co-authored-by: Giteabot <teabot@gitea.io>

2 files changed, 94 insertions, 134 deletions

diff --git a/web_src/css/shared/flex-list.css b/web_src/css/shared/flex-list.css
new file mode 100644
index 0000000000..7982241684
--- /dev/null
+++ b/web_src/css/shared/flex-list.css
@@ -0,0 +1,94 @@
+.flex-list {
+  list-style: none;
+}
+
+.flex-item {
+  display: flex;
+  gap: 8px;
+  align-items: flex-start;
+}
+
+.flex-item:not(:last-child) {
+  padding-bottom: 8px;
+}
+
+.flex-item-baseline {
+  align-items: baseline;
+}
+
+.flex-item-center {
+  align-items: center;
+}
+
+.flex-item .flex-item-leading {
+  display: flex;
+  align-items: flex-start;
+}
+
+.flex-item .flex-item-main {
+  display: flex;
+  flex-direction: column;
+  flex-grow: 1;
+  min-width: 0;
+}
+
+.flex-item-header {
+  display: flex;
+  gap: .25rem;
+  justify-content: space-between;
+  flex-wrap: wrap;
+}
+
+.flex-item a:not(.label, .button):hover {
+  color: var(--color-primary) !important;
+}
+
+.flex-item .flex-item-icon svg {
+  margin-top: 1px;
+}
+
+.flex-item .flex-item-trailing {
+  display: flex;
+  gap: 0.5rem;
+  align-items: center;
+  flex-grow: 0;
+  flex-wrap: wrap;
+  justify-content: end;
+  flex-shrink: 2;
+}
+
+.flex-item .flex-item-title {
+  display: inline-flex;
+  flex-wrap: wrap;
+  align-items: center;
+  gap: .25rem;
+  max-width: 100%;
+  color: var(--color-text);
+  font-size: 16px;
+  min-width: 0;
+  font-weight: var(--font-weight-semibold);
+}
+
+.flex-item .flex-item-title a {
+  color: var(--color-text);
+  overflow-wrap: anywhere;
+}
+
+.flex-item .flex-item-body {
+  font-size: 13px;
+  display: flex;
+  align-items: center;
+  flex-wrap: wrap;
+  gap: .25rem;
+  color: var(--color-text-light-2);
+}
+
+.flex-item .flex-item-body a {
+  color: inherit;
+  overflow-wrap: anywhere;
+}
+
+.flex-list > .flex-item + .flex-item {
+  border-top: 1px solid var(--color-secondary);
+  padding-top: 8px;
+}
diff --git a/web_src/css/shared/issuelist.css b/web_src/css/shared/issuelist.css
deleted file mode 100644
index 82fafedec0..0000000000
--- a/web_src/css/shared/issuelist.css
+++ /dev/null
@@ -1,134 +0,0 @@
-.issue.list {
-  list-style: none;
-  margin-top: 1rem;
-}
-
-.issue.list .item {
-  display: flex;
-  align-items: baseline;
-  padding: 8px 0;
-}
-
-.issue.list .item .issue-item-left {
-  display: flex;
-  align-items: flex-start;
-}
-
-.issue.list .item .issue-item-main {
-  display: flex;
-  flex-direction: column;
-  width: 100%;
-}
-
-.issue.list .item .issue-item-header {
-  display: flex;
-  justify-content: space-between;
-  align-items: center;
-  flex-wrap: wrap;
-}
-
-.issue.list a:not(.label):hover {
-  color: var(--color-primary) !important;
-}
-
-.issue.list > .item .issue-item-icon svg {
-  margin-right: 0.75rem;
-  margin-top: 1px;
-}
-
-.issue.list .item .issue-item-right {
-  display: flex;
-  gap: 0.5rem;
-}
-
-.issue.list > .action-item {
-  align-items: normal;
-}
-
-.issue.list > .item .action-item-center {
-  display: flex;
-  align-items: center;
-  padding-left: 4px;
-  padding-right: 12px;
-}
-
-.issue.list > .item .action-item-right {
-  flex: 0 0 15%;
-  display: flex;
-  flex-direction: column;
-  gap: 3px;
-  color: var(--color-text-light);
-}
-
-.issue.list > .item .issue-item-title {
-  max-width: 100%;
-  color: var(--color-text);
-  font-size: 16px;
-  min-width: 0;
-  font-weight: var(--font-weight-semibold);
-}
-
-.issue.list > .item .issue-item-title a.index {
-  max-width: fit-content;
-  display: -webkit-box;
-  -webkit-box-orient: vertical;
-  -webkit-line-clamp: 2;
-  overflow: hidden;
-  word-break: break-all;
-}
-
-.issue.list > .item .title {
-  color: var(--color-text);
-  overflow-wrap: anywhere;
-}
-
-.issue.list > .item .issue-item-body {
-  font-size: 13px;
-  display: flex;
-  align-items: center;
-  flex-wrap: wrap;
-  gap: .25rem;
-  color: var(--color-text-light-2);
-}
-
-.issue.list > .item .issue-item-body a {
-  color: inherit;
-  word-break: break-word;
-}
-
-.issue.list > .item .issue-item-body .checklist progress {
-  margin-left: 2px;
-  width: 80px;
-  height: 6px;
-  display: inline-block;
-  border-radius: 3px;
-}
-
-.issue.list > .item .issue-item-body .checklist progress::-webkit-progress-value {
-  background-color: var(--color-secondary-dark-4);
-}
-
-.issue.list > .item .issue-item-body .checklist progress::-moz-progress-bar {
-  background-color: var(--color-secondary-dark-4);
-}
-
-.issue.list .branches {
-  display: inline-flex;
-}
-
-.issue.list .branches .branch {
-  background-color: var(--color-secondary-alpha-40);
-  border-radius: 3px;
-  padding: 0 4px;
-}
-
-.issue.list .branches .truncated-name {
-  white-space: nowrap;
-  overflow: hidden;
-  text-overflow: ellipsis;
-  max-width: 10em;
-}
-
-.issue.list > .item + .item {
-  border-top: 1px solid var(--color-secondary);
-}