fix hardcoded AUTH_USER_MODEL in migrations

[joeyorlando]

Hi there 👋

Great project! I wanted to contribute to fixing a "bug" I realized in the migrations. When you set USER_MODEL as such in your settings.py:

PUSH_NOTIFICATIONS_SETTINGS = {
    "USER_MODEL": "user_management.User",
   ...
}

the migrations still create a foreign key reference to the value of AUTH_USER_MODEL.

This PR would update that to use the value of settings.PUSH_NOTIFICATION_SETTINGS["USER_MODEL"]

[joeyorlando]

It seems like this may be a duplicate of #572 .. but any way one of these can be merged in and released? 😄

[joeyorlando]

closing in favour of #572