The columns 'Created on' and 'Last activity' have been
added to the admin -> users view.

Sorting options have also been added for last activity
and the search bar has been moved to match the issues
page.

Whenever we already deal with a User object, let's use the more specific
method avatar_icon_for_user.

- Add a `destroy_user` ability. This didn't exist before, and was implicit in
  other abilities (only admins could access the admin area, so only they could
  destroy all users; a user can only access their own account page, and so can
  destroy only themselves).

- Grant this ability to admins, and when the current user is trying to destroy
  themselves. Disallow destroying ghost users in all cases.

- Modify the `Users::DestroyService` to check this ability. Also check it in
  views to decide whether or not to show the "Delete User" button.

- Add a short summary of the Ghost User to the bio.

This reverts commit bf2a86b7.

This reverts commit 68155ee7, reversing
changes made to 7ebd011e.