Add ReviewedVersionTracker
Ensure that ForeignKeys to proxy models work

Also gives ReviewedVersionTracker the right app_label
Fix misuse of super
Add interface for review queue
Enable the review queue on the demo site
Fix approved_by migration in order to support custom user model
Update demo/settings.py in order to support review queue
Use models.PROTECT for approvers
Fix “save and continue editing” button
Add undo button for approved individual commit