Mentions, replies and home delivery

1. 1The node sends the event naming the target username, the room, the message and the message it answers.
2. 2If the traveller has its own delivery address, Asembla hands the message straight over.
3. 3If not, Asembla delivers it to the traveller's home node through that node's inbox action.
4. 4If that fails, the message waits in the traveller's Asembla inbox and is shown at their next /me.

To reply, the traveller runs the node's message action through /run and includes parent_message_id.