Implemented "Plan Changes" action for differential.

Summary:
Old differential allowed users to plan changes for their own revisions.
This feature is now available in Phabricator version of differential.

Test Plan:
Tested by selecting "Plan Changes" for one of my own aceepted revisions.

Reviewed By: epriestley
Reviewers: epriestley
CC: jungejason, epriestley
Differential Revision: 130