Bug 23803: Add Font Awesome icon to cart in staff interface

Bug 23803: Add Font Awesome icon to cart in staff interface

This patch adds the font awesome icon to the cart in the staff
interface.

To test:

1. Apply patch and run yarn build (see:
 https://wiki.koha-community.org/wiki/Working_with_SCSS_in_the_OPAC_and_staff_client)

2. Check that the old cart icon no longer displays and the new font
   awesome icon is positioned correctly

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>