Fixes issues where you could only migrate forwards for subapp metrics,

Now I can develop stuff on master again!

Note that if you had run the migrations before, you need to drop the
metris.metrics table. Exactly how that's done, I'm still not sure. (I
moved models.py to models.py.backup, made an empty models.py file, and
then asked schemamigration to make me a migration, which I then ran)