Define bulk edits in terms of EditEngine, not hard-coded ad-hoc definitions
Only text fields -- and, specifically, only the "Title" field -- are supported after this change. Followup changes will add more bulk edit parameter types and broader field support.
However, the title field now works without any Maniphest-specific code, outside of the small amount of binding code in the ManiphestBulkEditor subclass.
Test Plan: Used the bulk edit workflow to change the titles of tasks.
Reviewed By: amckinley
Differential Revision: https://secure.phabricator.com/D18863