[ticket/11921] Improve appearance of Notifications in linklist

PHPBB3-11921
[ticket/11921] Improve appearance of PM notifications in linklist

PHPBB3-11921
[ticket/11921] Remove "l_" from non-lang variables

PHPBB3-11921
[ticket/11921] Move any HTML for Notifications to the templates
PHPBB3-11921
[ticket/11921] Only get the count of unread PMs
PHPBB3-11921
[ticket/11921] Directly inject unread messages count into the template

PHPBB3-11921
Merge pull request #1781 from VSEphpbb/ticket/11921

Ticket/11921 - Streamline PMs and Notifications in Header Link List