- 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.
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