[WIP] [Ticket/12888] Combine icons into single sprite

TODOs:
- [ ] Fix target unread & latest unread
- [ ] Finalize icons
- [ ] Cleanup deprecated code

Compatability
- [x] Safari
- [x] Chrome
- [x] Firefox
- [x] IE All
- [ ] IOS
- [ ] Android All

PSD: https://www.dropbox.com/s/6awigiemq4kv10q/navigation-icons.psd
Ticket: https://tracker.phpbb.com/browse/PHPBB3-12888

PHPBB3-12888