Bug 7687 - Remove problematic CSS gradient declarations targeting Internet Explorer

Bug 7687 - Remove problematic CSS gradient declarations targeting Internet Explorer

Removing and revising some CSS which causes problems in
Internet Explorer:

- User sidebar menu tabs in the OPAC
- Sidebar menu tabs in the staff client (circ, catalog, etc)
- List edit/delete buttons in the OPAC (when viewing the
  list of lists and viewing a list itself).

Also fixed is the sprite positioning for the "new list"
link which showed the wrong icon.

Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
I tested this in Firefox, and Chromium and it didn't break anything,
I trust it works for IE too.

Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>
Signed-off-by: Jared Camins-Esakov <jcamins@cpbibliography.com>