Fix initial migration for custom user model

Deleting a Device when having a custom user model and using south

The many to many relationships between the devices and the users are stored in the table "ios_notifications_device_users" of which the columns are "id", "device_id" and "user_id" are hard coded in the initial migration.

When using a custom user model, Django runtime will not be able to understand the m2m relationship : it will look for a column of a different name than "user_id".

As a consequence, deleting a device from the admin will result in a server error.