In the DM guessing code, prefer the oldest joined member if there's
anyone in the rom other than us. Otherwise, fall back to the old
behaviour.
Fixes https://github.com/vector-im/riot-web/issues/4288
- Set the verb for the people section to "tag as direct chat". This requires some CSS modifications to Riot because it's a long bit of text relative to, say, "demote".
- Because it's quite useful to be able to set the DM status of a room with just a boolean, add a convenience function for guessing a DM member and setting the DM flag on that room with the resulting member.
and mark it as one if so.
Also change the heuristic to only count rooms with 2 total members rather than 2 joined members, otherwise this is going to mark any room as a DM if someone creates a room, invites a bunch of people and you happen to be first to join.
