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.
8a2fec6f18