Offer option to Mute Forum instead of leaving it
If a user leaves a forum they've shared with others, we should warn the user that the people she has invited may be disconnected (or cut off) from the forum and do not receive any more messages from others. So we should ask the user to confirm. (Done in !164 (merged))
We could also offer them the option of "muting" the forum (i.e. turning off notifications) but remaining subscribed for the benefit of the people they've invited.
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