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>

tests/integration/admin_user_test.go

@@ -51,8 +51,8 @@ func testSuccessfullEdit(t *testing.T, formData user_model.User) {
 
 func makeRequest(t *testing.T, formData user_model.User, headerCode int) {
 	session := loginUser(t, "user1")
-	csrf := GetCSRF(t, session, "/admin/users/"+strconv.Itoa(int(formData.ID)))
-	req := NewRequestWithValues(t, "POST", "/admin/users/"+strconv.Itoa(int(formData.ID)), map[string]string{
+	csrf := GetCSRF(t, session, "/admin/users/"+strconv.Itoa(int(formData.ID))+"/edit")
+	req := NewRequestWithValues(t, "POST", "/admin/users/"+strconv.Itoa(int(formData.ID))+"/edit", map[string]string{
 		"_csrf":      csrf,
 		"user_name":  formData.Name,
 		"login_name": formData.LoginName,
@@ -72,7 +72,7 @@ func TestAdminDeleteUser(t *testing.T) {
 
 	session := loginUser(t, "user1")
 
-	csrf := GetCSRF(t, session, "/admin/users/8")
+	csrf := GetCSRF(t, session, "/admin/users/8/edit")
 	req := NewRequestWithValues(t, "POST", "/admin/users/8/delete", map[string]string{
 		"_csrf": csrf,
 	})