[ticket/12821] Fix All gradients

This is two changes
1. Fix all the current gradients to be consistent and handled correctly
2. Replace the remaining gradient gifs with css gradients IE: .headerbar, .forabg, .forumbg, .button1-3 , .forums, .dropdown-extended .header

TODO:
- [x] Chrome (Mac)
- [x] Firefox (Mac)
- [x] Safari (Mac)
- [ ] Opera (Mac)
- [x] Chrome (Win)
- [x] Firefox (Win)
- [ ] Opera (Win)
- [x] IE 8
- [x] IE 9
- [x] IE 10
- [x] Safari (IOS)
- [x] Chrome (IOS)
- [ ] Android

Ticket: https://tracker.phpbb.com/browse/PHPBB3-12821

PHPBB3-12821
@prototech The unfortunate thing about this change is it will make Prosilver now look different than it is supposed to, squaring off most, if not all, of its corners for the first time in Prosilver's life, in a browser (IE9) that is still technically supported by 3.1. Fallbacks to even older IE browsers (8 is still supported) is even worse, with gradients going away too. This PR may be biting off more than it can chew by creating major inconsistency in prosilver's appearance in two browsers still supported by phpBB 3.1, all for the sake of using CSS gradients instead of 4 little images!!