Handle conflicting offer migrations

Both the master branch and Django 1.7 branch introduced a South
migration for offers. It's been addressed by dropping the one introduced
on the Django 1.7 branch, and re-creating it.

The native migrations have been completely recreated.
Move app configs into Django's recommended apps.py

Django suggests having app configs in apps.py:
https://docs.djangoproject.com/en/dev/ref/applications/#for-application-authors

As Oscar's Application classes and AppConfigs might be merged at some
point in the future, the confusion of having both an app.py and apps.py
weighs less than the renaming pains for users when they get merged.