Description of feature: When a user replies to a MUC invitation with https://xmpp.org/extensions/xep-0045.html#invite-mediated <decline/> then any affiliation they have with the MUC should be removed as they have declined to join. Motivation: (Why?) Often (especially in a members only room) a potential member is given affiliation before they are even aware the MUC exists, either by an admin doing a grant or else by a side effect of a mediated invitation. If they decline this invitation they are then left as a "zombie" in the affiliation list until an admin manually removes them. This change would allow them to decline the invitation using the existing protocol, and have the list cleaned up properly.