fix-autojoin: filter instances in database
Move fix_autojoin core from admin controller to user model

Note that this slows down the fix-autojoin admin action; but the usage
of this function is extraordinary anyway, so I don't think this matters.
Autojoin newly activated users (fixes #907)
commit after User.fix_autojoin
avoid some commits in fix_autojoin admin action
refactor autojoin in User.create
Merge pull request #911 from nidico/feature-907-fix-user-import-autojoin

Fix autojoin on newly imported users