API for editing edge data
Summary: Add the `PhabricatorEdgeDataEditor` that allows you to edit the data attached to an edge and its inverse.
Test Plan: Create several edges and update the data attached. Once duplicating the data on both the edge and its inverse, and once setting different value for both.
Reviewers: epriestley
Reviewed By: epriestley
CC: aran, Korvin
Maniphest Tasks: T1279
Differential Revision: https://secure.phabricator.com/D5447