stop putting invite user objects in the session

the invited_user objects can be arbitrarily large, and when we put them
in the session we risk going over the session cookie's 4kb size limit.
since https://github.com/alphagov/notifications-admin/pull/3827 was
merged, we store the user id in the session. Now that's been live for a
day or two we can safely stop putting the rich object in the session.

Needed to change a bunch of tests for this to make sure appropriate
mocks were set. Also some tests were accidentally re-using fake_uuid.

Still pop the object when cleaning up sessions. We'll need to remove
that in a future PR.

tests/app/main/views/test_verify.py

@@ -176,6 +176,7 @@ def test_activate_user_redirects_to_service_dashboard_if_user_already_belongs_to
     api_user_active,
     mock_login,
     mock_get_service,
+    mock_get_invited_user_by_id,
 ):
     mocker.patch('app.user_api_client.add_user_to_service', side_effect=HTTPError(
         response=Mock(
@@ -189,10 +190,10 @@ def test_activate_user_redirects_to_service_dashboard_if_user_already_belongs_to
     ))
 
     # Can't use `with client.session_transaction()...` here since activate_session is not a view function
-    flask_session['invited_user'] = sample_invite
+    flask_session['invited_user_id'] = sample_invite['id']
 
     response = activate_user(api_user_active['id'])
 
     assert response.location == url_for('main.service_dashboard', service_id=service_one['id'])
 
-    flask_session.pop('invited_user')
+    flask_session.pop('invited_user_id')