Initial support for #25680

This PR only adds some simple styles from GitHub, it is big enough and
it focuses on adding the necessary framework-level supports. More styles
could be fine-tuned later.

I think it's better if the primary actions have primary color instead of
green which fits better into the overall single-color UI design. This PR
currently replaces every green button with primary:

<img width="141" alt="Screenshot 2023-09-16 at 14 07 59"
src="https://github.com/go-gitea/gitea/assets/115237/843c1e50-4fb2-4ec6-84ba-0efb9472dcbe">
<img width="161" alt="Screenshot 2023-09-16 at 14 07 51"
src="https://github.com/go-gitea/gitea/assets/115237/9442195a-a3b2-4a42-b262-8377d6f5c0d1">

Modal actions now use uncolored/primary instead of previous green/red
colors. I also removed the box-shadow on all basic buttons:

<img width="259" alt="Screenshot 2023-09-16 at 14 16 39"
src="https://github.com/go-gitea/gitea/assets/115237/5beea529-127a-44b0-8d4c-afa7b034a490">
<img width="261" alt="Screenshot 2023-09-16 at 14 17 42"
src="https://github.com/go-gitea/gitea/assets/115237/4757f7b2-4d46-49bc-a797-38bb28437b88">

The change currently includes the "Merge PR" button, for which we might
want to make an exception to match the icon color there:

<img width="442" alt="Screenshot 2023-09-16 at 14 33 53"
src="https://github.com/go-gitea/gitea/assets/115237/993ac1a5-c94d-4895-b76c-0d872181a70b">

Co-Author: @wxiaoguang 

This is the first step of #24229. And this PR will only includes html
changes, and followed by other PRs that fine tune css and change to
submenus.

After:

Admin Level

<img width="1400" alt="Screen Shot 2023-04-21 at 10 07 16"
src="https://user-images.githubusercontent.com/17645053/233523870-f848b61d-056a-4b41-9760-a9a49fea1fe8.png">

User Level

<img width="1422" alt="Screen Shot 2023-04-21 at 10 07 23"
src="https://user-images.githubusercontent.com/17645053/233523878-979adb20-a657-43d9-99a6-ad414010c0ef.png">

Repo Level

<img width="1404" alt="Screen Shot 2023-04-21 at 10 07 07"
src="https://user-images.githubusercontent.com/17645053/233523863-337440bd-c03a-4dfd-87fa-cef40300bfe0.png">

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

Previously, this setting was pretty confusing for users, especially the
difference between "reference" and "issue reference".
Related: #21321.

* Add main aria landmark to templates
 * Adjust some titles to improve understanding of location in navigation

Contributed by @Forgejo

* Refactor `i18n` to `locale`

- Currently we're using the `i18n` variable naming for the `locale`
struct. This contains locale's specific information and cannot be used
for general i18n purpose, therefore refactoring it to `locale` makes
more sense.
- Ref: https://github.com/go-gitea/gitea/pull/20096#discussion_r906699200

* Update routers/install/install.go

* Add config option to hide issue events
Adds a config option `HIDE_ISSUE_EVENTS` to hide most issue events (changed labels, milestones, projects...) on the issue detail page.
If this is true, only the following events (comment types) are shown:
* plain comments
* closed/reopned/merged
* reviews

* Make configurable using a list

* Add docs

* Add missing newline

* Fix merge issues

* Allow changes per user settings

* Fix lint

* Rm old docs

* Apply suggestions from code review

* Use bitsets

* Rm comment

* fmt

* Fix lint

* Use variable/constant to provide key

* fmt

* fix lint

* refactor

* Add a prefix for user setting key

* Add license comment

* Add license comment

* Update services/forms/user_form_hidden_comments.go

Co-authored-by: Gusted <williamzijl7@hotmail.com>

* check len == 0

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: Gusted <williamzijl7@hotmail.com>
Co-authored-by: 6543 <6543@obermui.de>

* Add appearance section in settings

* Fix lint

* Fix lint

* Apply suggestions from code review

Co-authored-by: Lauris BH <lauris@nix.lv>

Co-authored-by: Lauris BH <lauris@nix.lv>