User details page (#26713)

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

---------

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

templates/admin/layout_head.tmpl

@@ -1,6 +1,6 @@
 {{template "base/head" .ctxData}}
 <div role="main" aria-label="{{.ctxData.Title}}" class="page-content {{.pageClass}}">
-	<div class="ui container">
+	<div class="ui container gt-mb-4">
 		{{template "base/alert" .ctxData}}
 	</div>
 	<div class="ui container flex-container">