Add makemigrations check to CI

[stevejalim]

• We should run a CI check for a non-zero return from manage.py makemigrations --check
• We should add a similar check to our pre-commit hooks - see https://johnnymetz.com/posts/check-django-migrations/ for one way

[stevejalim]

This is slightly blocked on the challenge that if the basket data has changed since the commit was made, we'll get a failed migration check. Whether that should cause a fail is TBD, because it doesn't really block the behaviour of the system.