Insert Maniphest transactions when edges are edited

Summary:

• See D2741.
• When EdgeEditor performs edits, emit events.
• Listen for Maniphest edge events and save the changes as transactions.
• Do all this in a reasonably generic way that won't take too much rewriting as we use edges more generally.

Test Plan: Attached and detached commits from tasks, saw reasonable-looking transactions spring into existence.

Reviewers: btrahan, davidreuss

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D2906