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