[ticket/12442] Add CSS classes to important headings

PHPBB3-12442
[ticket/12442] More heading classes

I think that about finishes the most important one

PHPBB3-12442
Merge remote-tracking branch 'remotes/PayBas/ticket/12442' into develop-ascraeus

# By PayBas
# Via PayBas
* remotes/PayBas/ticket/12442:
  [ticket/12442] More heading classes
  [ticket/12442] Add CSS classes to important headings
Merge branch 'develop-ascraeus' into develop

# By PayBas
# Via Nathaniel Guse (1) and PayBas (1)
* develop-ascraeus:
  [ticket/12442] More heading classes
  [ticket/12442] Add CSS classes to important headings