Bug 23956: Replace famfamfam calendar icon in staff client with CSS data-url

Bug 23956: Replace famfamfam calendar icon in staff client with CSS data-url

This patch modifies the staff client CSS so that the backround image for
datepicker input fields is defined as a data-url rather than a reference
to an image file.

To test, apply the patch and regenerate the staff client CSS
(https://wiki.koha-community.org/wiki/Working_with_SCSS_in_the_OPAC_and_staff_client).

 - In the staff client, view any form which includes a datepicker-styled
   input. For instance, patron entry: date of birth, expiry; Aquisitions
   -> Late orders -> Filter results.
 - Confirm that the calendar icon looks correct.

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Joy Nelson <joy@bywatersolutions.com>