Check for missing migrations in `make check`

[jaredlockhart]

When we run make check we should also check that no migrations should have been created that were missed.

[jaredlockhart]

Looks like we can do python manage.py makemigrations --check and it will raise non-zero exit if it creates a migration.

[mythmon]

Prior art in the form of a Normandy PR to do the same thing: mozilla/normandy#1591