Make creating migrations more explicit

The test environment might test things like a custom User model, overriding apps, etc. and is generally allowed to be "dirty". For that reason, the ``create_migrations.py`` helper was slightly dodgy because it created migrations against the test environment instead of a truly vanilla Oscar environment. 

To make the process more explicit, I've dropped the migrations helper and added docs on how to create migrations against the sandbox. I've also disabled the custom user app in the sandbox to make it a vanilla Oscar environment.