Add a 'silent' option to differential.createcomment

Summary: See T1677. I think wanting bots to be able to post comments without sending email is a pretty reasonable use case. Eventually we should probably support this more broadly and maybe protect it with permissions (normal users maybe shouldn't be able to do this?) but we can wait for use cases.

Test Plan: Made comments with and without "silent". Verified that the non-silent comment sent email, and the silent comment did not.

Reviewers: btrahan, vrana

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1677

Differential Revision: https://secure.phabricator.com/D3341