fix: default for CertificatesDisplayBehaviors is enum string value, not enum.
fix: default for CertificatesDisplayBehaviors is enum string value, not enum.

This should correct a large number of errors encountered when trying
to dump courses to neo4j.

TNL-8386

edx/edx-platform,eduNEXT/edx-platform,edx/edx-platform,angelapper/edx-platform,eduNEXT/edx-platform,angelapper/edx-platform,edx/edx-platform,eduNEXT/edx-platform,edx/edx-platform,arbrandes/edx-platform,arbrandes/edx-platform,arbrandes/edx-platform,eduNEXT/edx-platform,arbrandes/edx-platform,angelapper/edx-platform,angelapper/edx-platform