Bridge detects only the first management room automatically

This is not a severe bug but confuses users because the syntax of bot commands change depending of if it's a first or second management room. Steps to reproduce: 1. Have a management PM room with Mautrix-Slack Bridge 2. Leave the room 3. Start a new PM with the bridge 4. Bridge tells: `This is not your management room: prefixing commands with !slack is required.` What should happen: The bridge should either: 1. Detect user leaving the PM room (the bot is alone in a room) and destroys the room automatically. 2. Accept all rooms created with `m.direct` hint as management rooms.