LP1998969 Make disabled <option>s more obvious

LP1998969 Make disabled <option>s more obvious

Changes the color of disabled options in <select> dropdowns to 30% gray.

To test, visit the staff catalog. In the search filters, with the search
type set to keyword, notice that the "matches exactly" and "starts with"
options in the "Contains" dropdown are now better distinguished from
valid choices.

Signed-off-by: Stephanie Leary <stephanie.leary@equinoxOLI.org>
Signed-off-by: Susan Morrison <smorrison@georgialibraries.org>
Signed-off-by: Jane Sandberg <sandbergja@gmail.com>