I was writing a topicLocalInvite handler when I realized how cumbersome
it's becoming, to have separate handlers for local activities. While it
allows me to pick custom specific message names and parameters (which is
why I took that approach in the first place), it causes a lot of
duplication and complexity (because I have to write the remote-activity
handlers anyway; adding local ones doesn't reduce complexity).
So this commit switches the entire system to communicate only using
AP/FF activities, including between local actors.
