Send invitations to users and groups

[evdherberg]

This PR adds functionality for sending invitations to users and groups. In two distinct ways: whenever a mutation is performed that causes new affiliations to exist, and explicitly by calling a specialised endpoint to send invitation messages.

Sending invitations is always optional, and the default is to not send any invitations.

Fixes #128.
Fixes #129.

[guusdk]

Will this work with the new endpoint that creates multiple rooms at once, as introduced by #134 ?

[guusdk]

Will this work with the new endpoint that creates multiple rooms at once, as introduced by #134 ?

It didn't, I think. As I was rebasing this to the current Main branch, I got a merge conflict. By fixing that, I introduced the fix required for that.

[guusdk]

@Fishbowler we force-pushed at about the same time. Yours seems to have gone in slighty before mine, which means that I've just overwritten your changes.

[Fishbowler]

Diff'd. Yours were better changes anyway 👍

[Fishbowler]

The "newly" bit of "newly affiliated users" doesn't appear to be working. When I call the room update endpoint twice with the same payload and query string, I get invitations both times.

[evdherberg]

New commit introduces some basic analysis of which affiliations are actually newly new in an update scenario. This should fix the problem: I don't get an invitation anymore the second time around.