Add support for SMTP-over-SSL.

Summary:
This is needed to support some backends (looking at you, GoDaddy) that don't
support STARTTLS. We just see if the port is 465, and use smtplib.SMTP_SSL
instead of smtplib.SMTP if so.

Fixes T661.

Test Plan:
Basic unit test added. Manually check that sending works via either
mechanism on backends that support both.

Reviewers: spang, kav-ya

Reviewed By: kav-ya

Maniphest Tasks: T661

Differential Revision: https://review.inboxapp.com/D905