This MR is based on the work submitted in !7 (closed), which automatically invites members of the moderation room. The original contributor has deleted their gitlab accoumt, so here's a new MR!
This MR also does a couple of extra things:
It fixes the bug that comes from trying to serialize a python set into JSON.
Invite functionality is now (superfically) covered by the unit tests. However, it is just the happy path in a simple scenario.