[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
1. cache is not a 100% answer to server requests, as it still has to load the image even from the cache. 
2. not providing this for those who do have modern browsers is a larger fault as we can always include the image for IE9 if absolutely necessary which is the only browser that has issue
3. As I said before their are four gradients being included, two of which work fine on all browsers, their is also the rewrite of all the current gradients for consistent browser coverage as well