Bug 25727: (follow-up) Use CSS specificity rather than !important

Bug 25727: (follow-up) Use CSS specificity rather than !important

This patch adds specificty to the CSS which controls the "invalid"
appearance of a Select2 dropdown so that the use of "!important" can be
avoided. The patch also puts this style into our custom select2 CSS file
so that it is located with other Select2 style changes.

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