Remove admin/pm rooms when the matrix-side user leaves to allow server to GC

[Half-Shot]

Fixes #1207

The idea here is that the rooms serve no purpose once the Matrix user has left them, so we might as well leave the bridge users from the room too and then Synapse can garbage collect the room (as it is unjoinable). Of course, matrix-org/synapse#4720 needs to happen first.

[benparsons]

lgtm

[pacien]

If I understand this correctly, this makes ghost users leave rooms only when real Matrix users are leaving them.

Is this also handling the case in which the real Matrix user already has left the room in the past (before this patch), leaving some rooms with only ghost users?

[Half-Shot]

Is this also handling the case in which the real Matrix user already has left the room in the past (before this patch), leaving some rooms with only ghost users?

This would probably be better handled by a script that iterates the database, parts the ghost and deletes the entry. I can probably write one up in a separate PR.