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

---
summary: Serve avatars from HTTP
...

Introduction
============

This module serves avatars from local users who have one set in their
vCard, see XEP-0054 and XEP-0153.

Configuring
===========

Simply load the module.  Avatars are then available at
http://<host>:5280/avatar/<username>

    modules_enabled = {
        ...
        "http_avatar";
    }

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

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  trunk   Works
  0.11    Works
  0.10    Should work
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