Inviting users to groups is not idempotent and throws exceptions

[spantaleev]

This is somewhat related to #2633 and caused by the changes done in #3128.

Currently, if a user is already invited to a group, subsequent invites (done with a PUT to /_matrix/client/r0/groups/<community-id>/admin/users/invite/<invitee-id>) would return an HTTP 500 error and would spew some database exceptions in the logs, like:

psycopg2.IntegrityError: duplicate key value violates unique constraint "group_invites_g_idx"

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Since this is a PUT request, I believe the invitation handling should be idempotent and the API should just return the same response it returned the first time ({"state": "invited"} or whatever).

That is, the above exception (or rather self.database_engine.module.IntegrityError) can probably just get swallowed somewhere. Where that should go exactly (groups/groups_server.py or storage/group_server.py) and how to swallow it from a deferred, I'm not sure.

[erikjohnston]

(Though spaces will likely replace communities)

[clokep]

Communities/groups are being removed, see #11584.