Notification implementation for Differential

Summary: The notification implementation has been extended to Differential. Appropriate changes have been made to the Differential editors and Differential feed story.

Test Plan: Tested out various actions available for Differential and confirmed that the notifications get delivered correctly and feed is generated.

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: allenjohnashton, ddfisher, aran, Korvin

Differential Revision: https://secure.phabricator.com/D2696