Add support for custom welcome messages

[justinbot]

Spun out of mautrix/signal#139

Example configuration for a bridge to replace the generic default messages:

    # Messages sent upon joining a management room.
    # Markdown is supported. The defaults are listed below.
    management_room_text:
        # Sent when joining a room.
        welcome: "Hello, I'm a Signal bridge bot."
        # Sent when joining a management room and the user is already logged in.
        welcome_connected: "Use `help` for help."
        # Sent when joining a management room and the user is not logged in.
        welcome_unconnected: "Use `help` for help or `register` to log in."
        # Optional extra text sent when joining a management room.
        # additional_help: "This would be some additional text in case you need it."

    # Send each message separately (for readability in some clients)
    management_room_multiple_messages: false

[tulir]

Looks good, thanks!