ActivityPub integration tests and Django

In the following merge request is the implementation of a feature that requires a live server. It relies on APILiveServerTestCase and does not share anything with the pytest fixture based tests. It should be fine because it is one of a kind. It is not worth the effort to figure out how to make it DRY.

https://lab.fedeproxy.eu/fedeproxy/fedeproxy/-/merge_requests/50