- 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.
`ProjectTeam#find_member` doesn't take group links into account. It was
used in two places:
1. An admin view - it can stay here.
2. `ProjectTeam#member?`, which is often used to decide if a user has
access to view something.
This second part broke confidential issues viewing. `IssuesFinder` ends
up delegating to `Project#authorized_for_user?`, which does consider
group links, so users with access to the project via a group link could
see confidential issues on the index page. However, `IssuesPolicy` used
`ProjectTeam#member?`, so the same user couldn't view the issue when
going to it directly.
