feat(Mail): Use PHPMailer v5.2.8 instead of PEAR's Mail_Mime/Net_SMTP (refs #)(refs #4997)

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 #)