[WIP] Start of user accepting invite.

This commit only deals with acceptance by
users who are already in system.

Changed invite client to return invited user objects
instead of dictionaries.

Added commented out test. fixed up fixtures to return invited user
object for invites

tests/app/main/views/test_accept_invite.py

@@ -0,0 +1,25 @@
+from flask import url_for
+
+
+def test_existing_user_accept_invite_calls_api_and_redirects_to_dashboard(app_,
+                                                                          service_one,
+                                                                          sample_invite,
+                                                                          mock_accept_invite,
+                                                                          mock_get_user_by_email,
+                                                                          mock_add_user_to_service):
+
+    expected_service = service_one['id']
+    expected_from_user = service_one['users'][0]
+    expected_redirect_location = 'http://localhost/services/{}/dashboard'.format(expected_service)
+
+    with app_.test_request_context():
+        with app_.test_client() as client:
+
+            response = client.get(url_for('main.accept_invite', token='thisisnotarealtoken'))
+
+            mock_accept_invite.assert_called_with('thisisnotarealtoken')
+            mock_get_user_by_email.assert_called_with('invited_user@test.gov.uk')
+            mock_add_user_to_service.assert_called_with(expected_service,  expected_from_user, sample_invite)
+
+            assert response.status_code == 302
+            assert response.location == expected_redirect_location