R1:522e5b47795f

Build an event dispatch mechanism into Phabricator

Summary:
This is an attempt to satisfy a lot of the one-off requests a little more
generally, by providing a relatively generic piece of event architecture.

Allow the registation of event listeners which can react to various application
events (currently, task editing).

I'll doc this a bit better but I wanted to see if anyone had massive objections
to doing this or the broad approach. The specific problem I want to address is
that one client wants to do…