Support SMTP as the mailer.

Summary:
Support SMTP as the mailer and user could turn on SMTP authentication if needed.
Import PHPMailer as PHPMailerLite doesn't support SMTP.

Make class PhabricatorMailImplementationPHPMailerAdapter final.

Test Plan: N/A

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2139

Differential Revision: https://secure.phabricator.com/D4063