LP#1772680: enhancements to RBDigital integration

LP#1772680: enhancements to RBDigital integration

This feature allows searching of RBdigital resources from the Evergreen
catalog without needing to import and maintain bibliographic records for
the RBdigital resources.

The RBdigital search results will appear in the OPAC on a different tab -
a search in basic or advanced Evergreen search will carry over to the
RBdigital tab (along with filters when mapping is feasible) so that the
user can type in a search once and see both sets of results. The design
of the RBdigital tab will make use of the OPAC styling for consistency.

This feature is sponsored by Recorded Books (RBDigital) and
GPLS and was primary written by Jaswinder Singh with code contributions
and testing from the Equinox Open Library Initiative.

Signed-off-by: Jaswinder Singh <jaswinder.singh0011@gmail.com>
Signed-off-by: Galen Charlton <gmc@equinoxinitiative.org>