LP1970946 Adjust color contrast in result highlights

LP1970946 Adjust color contrast in result highlights

Lowers the contrast on search result match highlights in the staff
catalog, and changes the highlight tag from <b> (PostgreSQL's default),
to <mark> (new in HTML5).

As an additional consequence, the color contrast for highlighted
search terms in the Bootstrap OPAC (using default styles) improves
from 4.38 to 17.12.

Signed-off-by: Stephanie Leary <stephanie.leary@equinoxOLI.org>
Signed-off-by: Jane Sandberg <js7389@princeton.edu>
Signed-off-by: Galen Charlton <gmc@equinoxOLI.org>