[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
I agree that this should be done too - just not here. When a new style get conveived from the ground up, and decisions like using CSS for any gradients is made, the design can be made to accommodate that consistently across all browsers, which would mean no rounded borders at all in that case.

But here, Prosilver has been defined by its rounded borders for many years. We can't change that, in just one browser, because we are trying to switch from 4 images to CSS gradients. Those 4 images have worked very well all these years have not held back custom style development.