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>

routers/web/web.go

@@ -573,7 +573,8 @@ func registerRoutes(m *web.Route) {
 		m.Group("/users", func() {
 			m.Get("", admin.Users)
 			m.Combo("/new").Get(admin.NewUser).Post(web.Bind(forms.AdminCreateUserForm{}), admin.NewUserPost)
-			m.Combo("/{userid}").Get(admin.EditUser).Post(web.Bind(forms.AdminEditUserForm{}), admin.EditUserPost)
+			m.Get("/{userid}", admin.ViewUser)
+			m.Combo("/{userid}/edit").Get(admin.EditUser).Post(web.Bind(forms.AdminEditUserForm{}), admin.EditUserPost)
 			m.Post("/{userid}/delete", admin.DeleteUser)
 			m.Post("/{userid}/avatar", web.Bind(forms.AvatarForm{}), admin.AvatarPost)
 			m.Post("/{userid}/avatar/delete", admin.DeleteAvatar)