Add an option to put comment context into emails.

Summary:
When enabled, this will show the full history of review comments in an
email-compatible threading-view.

Test Plan: Sending emails with the option on and off.

Reviewers: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D10146
Address review comments.
Merge.