Force-join not properly implemented

When a client got desynchronized and joins a MUC, it sends a <presence><x/></p> stanza. The MUC service (biboumi in this case) should react by properly syncing the client, even if it is still listed as a participant.

Background: https://github.com/xsf/xeps/pull/499

Currently, the rejoining client will only receive partial presence updates, and won't be able to list all channel participants.

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