Remove email from handles

Summary:
Since user emails aren't in the user table, we had to do extra data fetching
for handles, and the emails are only used in MetaMTA, so we move the email
code into MetaMTA and remove it from handles.

Test Plan: send test emails

Reviewers: jungejason, vrana, epriestley

Reviewed By: epriestley

CC: aran, Koolvin

Differential Revision: https://secure.phabricator.com/D2494