Rooms representing Slack's "Multi-Person DMs" should be named the way Slack names them

[thomas-profitt]

A ghost Matrix room for a multi-person DM will be properly created, though not set as a direct-message room, but it will not be named properly.

If Larry is the user using the bridge, and Larry is in a Slack Multi-Person DM with Moe and Curly, in Slack the channel will show up as "Moe, Curly", but in Larry's Matrix client he'll see "mpdm-larry--moe--curly-1".