Show warning when no sessions are active
Also subscribe to credentialsUpdate to get an earlier opportunity to fiddle our BTrees, reducing conflicts
Merge branch 'master' into matthewwilkes-admin-force-logout

Conflicts:
	Products/NoDuplicateLogin/plugin.py
Key uuid_to_time on uuid, not login
Merge branch 'master' into matthewwilkes-admin-force-logout