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](1ad57ac9-f20a-45a4-8477-ffe572a41e9e)


![grafik](25786ecd-cb9d-4c92-90f4-e7f4292c073b)


</details>

Partially resolves #25939

---------

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

web_src/css/admin.css

@@ -42,3 +42,10 @@
 .admin .table th {
   white-space: nowrap;
 }
+
+.admin-responsive-columns {
+  display: flex;
+  flex-wrap: wrap;
+  gap: 1rem;
+  margin-bottom: 1rem;
+}