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

Contributing
============

Guidelines for developers
-------------------------

-   Each module should be contained in a folder of its name (e.g.\
     `mod_ping/mod_ping.lua`)
-   Each module should have a `README.markdown` page in their folder
    with a description, usage, configuration and todo sections (feel
    free to copy an existing one as a template)
-   Commit messages should begin with the name of the plugin they are
    for (e.g. `mod_ping: Set correct namespace on pongs`)

Instructions on cloning the repository are at
<https://prosody.im/doc/installing_modules#prosody-modules>