Implement `disco#info` for IRC channels
A disco#info query to an IRC channel should emulate https://xmpp.org/extensions/xep-0045.html#disco-roominfo and return something useful for both theoretical and for joined channels.
- return the channel name ("#foobarbaz") as the
<identity name=''>value, so that modern XMPP clients can use it in notifications instead of "#email@example.com"
- provide additional info (number of participants, subject) from a
/list #foobarbazif the channel is not joined
As an example, yaxim does display the channel name, description and number of participants if you get invited into a MUC - that might be useful here as well :)