Globally limit the size of generated emails.

Summary: At the global level, truncate emails at a user-configured size.

Test Plan: Untested, as I could not get PHP to send emails on my box, but if you can this should be very easy to test. Just set the max size to something like .001 kilobytes and make sure it does the right thing.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin, AnhNhan

Maniphest Tasks: T1392

Differential Revision: https://secure.phabricator.com/D6118