add public pipeline-preferred event table

gracedb/api/v1/superevents/serializers.py

gracedb/api/v1/superevents/tests/test_serializers.py

gracedb/templates/superevents/detail.html

...perevents/pipeline_preferred_event_info_table_public.html

...plates/superevents/preferred_event_info_table_public.html