Recover moved project avatar icons

Some project avatars have been moved/lost due to a bug when forking projects. It might be possible to recover these using a backup, or by using avatars from forks.

Alternatively we might just ask users to re-upload missing avatars in the release post.

See https://gitlab.com/gitlab-org/gitlab-ce/issues/26158#note_20818894

Posible solutions

• This could be a rake task or something which happens in the background automatically after installing the patch release
• On large installations it might have to do a file lookup for every project, and a file copy for every project which has been forked since the bug
• Could be limited to parents of recently forked repos, with a cutoff to exclude projects forked before the bug was introduced.
• Could use timestamps on icons to ignore icons uploaded since the fork was created