[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
The fact that the header isn't 92px doesn't really matter all that much. The gradient isn't required to "finish" at the exact bottom for any particular element.

I'm merely pointing out the fact this change will scale the gradient to the entire height, which is new behavior. 

If you take `.forabg` on the index for example, you can really see the difference (just like my screenshot above). I personally like the old look better because the dark color has the upper hand (providing more contrast). The same goes for gradient.gif (`ul.forums`) but in reverse; it looks significantly darker now.

But that is my personal preference.