Bug 6865 - Replace image-based gradient backgrounds with CSS3 gradients

Bug 6865 - Replace image-based gradient backgrounds with CSS3 gradients

Replaces image-based gradients in:

OPAC search bar
OPAC submit buttons
OPAC lists menu background
OPAC alert and message boxes
OPAC user menu tabs
Staff client alert and message boxes
Staff client side menu tabs

Removes obsoleted background image files.

Cross-browser background-gradients created using:
http://www.colorzilla.com/gradient-editor/

Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>