mod_vcard_muc: Add an alternative method of signaling avatar change When the avatar has been changed, a signal is sent that the room configuration has changed. Clients then do a disco#info query to find the SHA-1 of the new avatar. They can then fetch it as before, or not if they have it cached already. This is meant to be less disruptive than signaling via presence, which caused problems for some clients. If clients transition to the new method, the old one can eventually be removed. The namespace is made up while waiting for standardization. Otherwise it is very close to what's described in https://xmpp.org/extensions/inbox/muc-avatars.html

Introduction
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This module checks the IP address of newly registered users against a
DNS block list. If a positive match is found, it gets logged.

Configuration
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  Option              Type     Default
  ------------------- -------- ------------
  registration\_rbl   string   *Required*