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

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>