[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:
- [x] Chrome
- [x] Firefox
- [x] Safari
- [x] IE
- [x] IOS
- [ ] Android
- [x] Fix topic icon

Demo: http://12911.midaym.com/viewforum.php?f=2&sid=4d651a2a4d14afd7d883392bbc4985cc
Ticket: https://tracker.phpbb.com/browse/PHPBB3-12911

PHPBB3-12911
yes because icon is used for the header and the rows so one of them needs changed