Refactor filtering out accepted invites to client

None of our model or view layer code should need to know about accepted
invites. We don’t use them anywhere because once an invite is accepted
that person is now a user.

Putting this logic in the client means that:
- none of the code calling the client needs to care about accepted
  invites
- it’s easier to (if we want) update the API code to not return accepted
  invites

tests/conftest.py

@@ -2174,10 +2174,10 @@ def mock_get_invites_for_service(mocker, service_one, sample_invite):
         for i in range(0, 5):
             invite = copy.copy(sample_invite)
             invite['email_address'] = 'user_{}@testnotify.gov.uk'.format(i)
-            data.append(InvitedUser(**invite))
+            data.append(invite)
         return data
 
-    return mocker.patch('app.invite_api_client.get_invites_for_service', side_effect=_get_invites)
+    return mocker.patch('app.invite_api_client._get_invites_for_service', side_effect=_get_invites)
 
 
 @pytest.fixture(scope='function')