OF-1677: Add disco#info feature for self-ping
The updated XEP-0410 introduces a disco#info feature that is to be included in
responses sent from a room. This commit adds the feature (if enabled).
OF-1677: Allow self-ping feature to be disabled.
This introduces a property that lets an admin disable the newly introduces
'self-ping' functionality. It is configured using the 'xmpp.muc.self-ping.enabled'
property, of which the value is expected to be a boolean.
This is intended as a failsafe, in case the feature introduces some kind of bug.
