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>
</details>
Partially resolves #25939
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Co-authored-by: Giteabot <teabot@gitea.io>
