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
Add unapprove button

Unapprove button doesn't use UndoView because it would need to generate
a UndoApprovalsForm for every commit in the history view.