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

This module implements the [Server
Optimization](https://xmpp.org/extensions/xep-0410.html#serveroptimization)
part of [XEP-0410: MUC Self-Ping]

# Usage

The module is loaded on MUC components:

```lua
Component "muc.example.com" "muc"
modules_enabled = {
    "muc_ping";
}
```

# Configuration

No options.

# Compatibility

It should work with Prosody up until 0.10.x.

Prosody 0.11.x and trunk natively supports XEP-0410 so this module is **not** needed.