Ticket/12821

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

Ticket: https://tracker.phpbb.com/browse/PHPBB3-12821

PHPBB3-12821