Added autocomplet_fields to admin
Added autocomplet_fields to admin

The `user` field on `SentNotifications` makes the admin slow if you have 1000s of users.  Added `autocomplete_fields`.  This field would be ignored by Django < 2.0.

Caveat is that the `User` admin has to have `search_fields` defined for autocomplete to work. By default, `django.contrib.auth.admin.UserAdmin` has `search_fields = ('username', 'first_name', 'last_name', 'email')` so that shouldn't be a problem unless someone has created a custom admin for `User` and did not define `search_fields` in it.
worthwhile/django-herald,worthwhile/django-herald