Display participant names as muc name in groupchats if empty

As discussed with daniel earlier today.

The idea would be to display names of (some?) participants as name of the room, if the room name is empty, and the room is of type "groupchats"**. For example in a room with Link Mauve, Zash and me, the name could be generated as "Link Mauve, pep., and Zash".

There should probably be a length limit, with ellipsis. I am also not entirely sure about the order? As they come in from the participants request? Alphabetically?

**: still to be defined, but I guess along the lines of

• muc#roomconfig_memberonly: True
• muc#roomconfig_publicroom: False
• muc#roomconfig_whois: 'anyone'