Make new tests for user auth in backend

[sjsivert]

Tests to be rewritten:

test_exact_motive_search_retrieves_single_image
test_admin_user_can_see_all_photos
test_fg_user_can_see_all_photos
test_husfolk_user_can_only_see_husfolk_and_alle
test_power_user_can_see_husfolk_and_alle
test_anon_user_can_only_see_alle

[sjsivert]

seed.sh failer nå med feilmelding:
self.code(from_state.apps, schema_editor) File "/django/fg/api/migrations/dev_seed.py", line 114, in populate_users fg_user = User.objects.get(username='fg') File "/usr/local/lib/python3.6/site-packages/django/db/models/manager.py", line 82, in manager_method return getattr(self.get_queryset(), name)(*args, **kwargs) File "/usr/local/lib/python3.6/site-packages/django/db/models/query.py", line 399, in get self.model._meta.object_name __fake__.DoesNotExist: User matching query does not exist.