This change moves most of the logic related to starting and
stopping outbound typing indicators to a new module called
typing_status.js that is heavily unit tested.
While this was in some sense a rewrite, the logic was mostly
inspired by the existing code.
This change does fix one known bug, which is that when we
were changing recipients before (while typing was active), we
were not stopping and starting typing indicators. This was
a fairly minor bug, since usually users leave the compose
box to change recipients, and we would do stop/start under
that scenario. Now we also handle the case where the user
does not leave the compose box to change recipients.
Steve Howell