Added support for custom AUTH_USER_MODEL

Running migrations in a project that defines a custom user model results
in the error:

```
ValueError: Lookup failed for model referenced by field admin.LogEntry.user: accounts.User
```

More about the issue can be found at
https://code.djangoproject.com/ticket/22563.

Adding the `swappable_dependency()` dependency solves the issue allowing
the migration to run successfully.