Deleting the -internal- admin room causes the bridge to crash on startup

[tadzik]

Were a Synapse admin to delete the -internal- user admin room that we use for appservice pings here, the bridge will not attempt to create it, but it will also fail to join it – causing it to crash on startup and not recover after a restart.

Deleted rooms can be heuristically identified as having empty state, but I think we'd need to be Synapse admins to determine that. We need to somehow recognize this situation anyway, and reset the internal room.

The workaround for this occuring in the wild is a delete from user_admin_rooms where matrixuser = '-internal-'; on the bridge database.