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
