R1:6639f93153dc

Add a 'silent' option to diffusion.createcomment

Test Plan: Created comments with 'silent' both true and empty, received notifcation for only the latter.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D7916