New maniphest event TYPE_MANIPHEST_DIDEDITTASK

Summary:
This event is fired after a task is created and assigned with an id.
Use case is sending an email notification to everyone in a project when a new task is
submitted to said project.

Test Plan:
Implement the event listener, submit a new task to a project, see if the project members
receive an email notification. I will submit the event handler in a separate diff once it's a bit
prettier and tested more thoroughly.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, jungejason

Differential Revision: https://secure.phabricator.com/D2159