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

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summary: Cache caps on user sessions
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Description
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This module listens on presences containing caps (XEP-0115) and asks the client
for the corresponding disco#info if it changed.

It fires the c2s-capabilities-changed event once the disco#info result is
received.

Compatibility
=============

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  trunk   Works
  0.11    Does not work
  0.10    Does not work
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