MBS-5624: Show a banner if a release group is eligible for automatic deletion

The banner is shown if the release group has no edits pending,
is not involved in any open edits, has no releases and has no
relationships.
MBS-6779: Request apt-get install cpanminus in INSTALL.md
Merge branch 'mbs-6779' into beta
Merge branch 'mbs-5624' into beta