Add Subscribe option to maniphest

Summary:
Same as //Subscribe//, //Unsubscribe// and //Automatically Subscribed// in differential.

Manually updated library map as windows is fun!

Test Plan: Subscribe, Unsubscribe!

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5809