[ticket/12608] Improve notifications drop-down menu styling in header

PHPBB3-12608
[ticket/12608] Updated tests

PHPBB3-12608
[ticket/12608] Add styling (CSS class) options to notifications

This will allow different notifications to have different styles

PHPBB3-12608
[ticket/12608] Clean up whitespace

PHPBB3-12608
[ticket/12608] Improved LTR and various bugfixes

PHPBB3-12608
[ticket/12608] Remove somewhat redundant reported color style

PHPBB3-12608
[ticket/12608] Changed get_styling() to get_style_class()

PHPBB3-12608
[ticket/12608] Add subSilver

PHPBB3-12608
[ticket/12608] Change CSS classes to prevent styling conflicts

PHPBB3-12608
Merge remote-tracking branch 'PayBas/ticket/12608' into develop-ascraeus

* PayBas/ticket/12608:
  [ticket/12608] Change CSS classes to prevent styling conflicts
  [ticket/12608] Add subSilver
  [ticket/12608] Changed get_styling() to get_style_class()
  [ticket/12608] Remove somewhat redundant reported color style
  [ticket/12608] Improved LTR and various bugfixes
  [ticket/12608] Clean up whitespace
  [ticket/12608] Add styling (CSS class) options to notifications
  [ticket/12608] Updated tests
  [ticket/12608] Improve notifications drop-down menu styling in header