feat(Mail): Use PHPMailer instead of PEAR's Mail_Mime/Net_SMTP (refs #4759)(refs #4997)

Snapshot of PHPMailer's master branch:
- https://github.com/PHPMailer/PHPMailer/commit/e43b2649175d076e01a695c3897c547d89ca05ed

Also:
- Handle images with src="file/…" URLs in srcfile() from mailcard.php and
  Class.MailTemplate.php (refs #4997)
test(Dcp\Mail\MailAddrParser): Add test for Dcp\Mail\MailAddrParser (refs #4759)