This PR implements a proposal to clean up the admin users table by
moving some information out to a separate user details page (which also
displays some additional information).

Other changes:
- move edit user page from `/admin/users/{id}` to
`/admin/users/{id}/edit` -> `/admin/users/{id}` now shows the user
details page
- show if user is instance administrator as a label instead of a
separate column
- separate explore users template into a page- and a shared one, to make
it possible to use it on the user details page
- fix issue where there was no margin between alert message and
following content on admin pages

<details>

<summary>Screenshots</summary>


![grafik](https://github.com/go-gitea/gitea/assets/47871822/1ad57ac9-f20a-45a4-8477-ffe572a41e9e)


![grafik](https://github.com/go-gitea/gitea/assets/47871822/25786ecd-cb9d-4c92-90f4-e7f4292c073b)


</details>

Partially resolves #25939

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

Same as https://github.com/go-gitea/gitea/pull/26026 but for the user
settings page. It introduces a new `flex-container` class and shares it
across both pages.

Before and After:
<img width="1264" alt="Screenshot 2023-07-21 at 19 35 57"
src="https://github.com/go-gitea/gitea/assets/115237/1358dab4-55c0-40ce-a4d5-673099304f3d">
<img width="1269" alt="Screenshot 2023-07-21 at 19 35 42"
src="https://github.com/go-gitea/gitea/assets/115237/34812f6d-dc65-4009-b977-90e03efdc6d1">

This PR includes #26007 's changes but have a UI to prompt administrator
about the deprecated settings as well as the log or console warning.
Then users will have enough time to notice the problem and don't have
surprise like before.

<img width="1293" alt="图片"
src="https://github.com/go-gitea/gitea/assets/81045/c33355f0-1ea7-4fb3-ad43-cd23cd15391d">

---------

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

Reduce margins around admin boxes and reduce sidebar size from 275px to
240px. This is the same 16px margin we use on issue pages.

Before and After:
<img width="1270" alt="Screenshot 2023-07-21 at 00 28 11"
src="https://github.com/go-gitea/gitea/assets/115237/f9b0dcb0-8f7e-49b4-b130-54bf31c142fd">
<img width="1271" alt="Screenshot 2023-07-21 at 00 30 51"
src="https://github.com/go-gitea/gitea/assets/115237/ddd75d59-9ab9-4061-8989-852e89727560">

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>