[WIP] [Ticket/12911] convert imageset to sprite

convert all the forum/topic imageset icons into a single sprite to reduce server requests and overall file size

TODO:
- [] Chrome
- [] Firefox
- [] Safari
- [] IE
- [] IOS
- [] Android
- [] Fix topic icon

Demo: http://12911.midaym.com/viewforum.php?f=2&sid=4d651a2a4d14afd7d883392bbc4985cc
Ticket: https://tracker.phpbb.com/browse/PHPBB3-12911

PHPBB3-12911