- Use `GlobalPolicy` to authorize the users that a non-authenticated user can
fetch from `/api/v4/users`. We allow access if the `Gitlab::VisibilityLevel::PUBLIC`
visibility level is not restricted.
- Further, as before, `/api/v4/users` is only accessible to unauthenticated users if
the `username` parameter is passed.
- Turn off `authenticate!` for the `/api/v4/users` endpoint by matching on the actual
route + method, rather than the description.
- Change the type of `current_user` check in `UsersFinder` to be more
compatible with EE.
- 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.
