Auto invite

Asbjørn Olling requested to merge auto-invite into main

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.

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