Found while working on #2783

 1. Start a DM conversation
 1. Delete the bridged room entry: `mongo gitter` -> `db.matricesbridgedroom.remove({ troupeId: db.troupes.findOne({ lcUri: 'matrix/5f762f95986e461e663059d1/@root:my.matrix.host' })._id })`
 1. Chat in the DM conversation from the Gitter side again, see the app attempt to make a room for the DM using the wrong code path