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: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D10146