Only consider g-events that generate public alerts for pipeline preferred events

This MR adds a set named PUBLIC_NOTICES_GROUP_SEARCH_SET to the superevents task, which is a list of all of the group/search combinations that we send alerts for. This is then used by the select_pipeline_preferred_event task to make sure we only consider candidates that are approved for public notices. I've also added a unit test to make sure this list is in sync with the config file, and tested that changing the config file breaks the unit test (I did this by locally changing the cbc/ssm threshold to 1 / 30 days and rerunning the unit test).