Spoof usernames with Amazon SES

Summary:
When I tested this earlier I was incorrectly interpreting PHPMailer errors as
SES errors. This works fine as long as you get around the peculiarities of
PHPMailer.

Test Plan:
Sent email to myself, received email from a human-readable address in my mail
client.

Reviewed By: rm
Reviewers: rm, tuomaspelkonen, jungejason, aran
CC: aran, rm, epriestley
Differential Revision: 246