Enable migrations as required by Django 1.8+ ...

Django 1.8+ won't let an app without migrations depend on django.contrib.auth
models, which is migrations-enabled. Django will create the legacy tables first
and there will be a database error.
This commit add basic migrations support in Postorius by following:
https://docs.djangoproject.com/en/1.8/topics/migrations/#adding-migrations-to-apps