[ticket/8228] Fix whitespaces before code in Firefox

Based on the "[code] enhancements" modification by TerraFrost

PHPBB3-8228
[ticket/8228] Fix still existing problems with code in firefox

A user of a board mentioned that there are still some problems
in firefox if php-highlighting is on. So I used a snippet which
they worked on for weeks. Link to the snippet (in german):
http://www.ongray-design.de/forum/viewtopic.php?t=541

PHPBB3-8228
Merge remote-tracking branch 'Elsensee/ticket/8228' into develop

* Elsensee/ticket/8228:
  [ticket/8228] Fix still existing problems with code in firefox
  [ticket/8228] Fix whitespaces before code in Firefox