View CiviMail Mailings - page should include an HTML title if one isn't provided in the message body

Currently when you view a previously sent CiviMail mailing (civicrm/mailing/view) - there's no <title> in the document (unless one has been explicitly included in the mailing HTML body). This makes the view incomplete.

Add logic to set the HTML page title = mailing subject if a title has not already been provided in the message body.