get_user_model should not be used with ManyToManyField 

It may cause race conditions, with ios_notification being loaded before app that defines custom user.

From docs[0]:
> When you define a foreign key or many-to-many relations to the User model, you should specify the custom model using the AUTH_USER_MODEL setting. For example:

Related:
https://code.djangoproject.com/ticket/19753#comment:14

[0] https://docs.djangoproject.com/en/1.5/topics/auth/customizing/#django.contrib.auth.get_user_model