Bug 28819: (follow-up) Use stacked icons for different searches

Bug 28819: (follow-up) Use stacked icons for different searches

This patch modifies the staff interface home page so that the "Advanced
search" and "Item search" links are differentated using stacked Font
Awesome icons.

To test, apply the patch and view the staff interface home page. The
icons associated with advanced and item searches should be different:

- The Advanced search link should have a magnifying glass icon on top of
  a grey book icon.
- The Item search link should have a magnifying glass icon on top of a
  grey list icon.

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>