Removes filters as a way to provide gradients for IE<=9

Removes filters as a way to provide gradients for IE<=9
 - Lets us use rounded corners in IE9
 - Makes hover / active states work correctly in IE<=9

Only change for non-IE is that moving from hover -> not hovering
no longer animates (was a subtle blend) due to mismatch between the
bottom of the gradient and the background color.

Refactor: moves the primary button background to match the others.