Bug 27321: Make disabled DB columns clearly disabled visually

Bug 27321: Make disabled DB columns clearly disabled visually

To test:
1. Apply patch and its dependency (bug 17364)
2. Rebuild the CSS (https://wiki.koha-community.org/wiki/Working_with_SCSS_in_the_OPAC_and_staff_client)
3. Go the system pref BorroweUnwantedFields
4. branchcode should cleary stand out as disabled (#cccccc)

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>