[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
It is. We just don't want this simple PR turning into another long "which looks better" discussion. So I would suggest sticking as close to the current look as possible.

We can discuss possible changes to the look in the future.