We now isolate the code to transmit messages into transmit.js.
It is stable code that most folks doing UI work in compose.js don't
care about the details of, so it's just clutter there. Also, we may
soon have other widgets than the compose box that send messages.
This change mostly preserves test coverage, although in some cases
we stub at a higher level for the compose path (this is a good thing).
Extracting out transmit.js allows us to lock down 100% coverage on that
file.
Steve Howell