[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
Assuming that 3.2 will still support IE9, and proSilver 3.2 will still have some combination of gradients and rounded-corners, we're going to have to bite the bullet on this sooner or later.

Whether we push this to 3.2 or not doesn't matter much to me. It doesn't make any functional difference, other than that we slightly reduce the number of server requests (which is especially nice for mobile devices).