Superevents: show preferred event(s) by pipeline

via the AllSky LVK session:

Pipelines must be able to identify a single preferred trigger if multiple have been uploaded to GraceDB

This could be as simple as having a table on a superevent landing page that scrapes the min-far/snr candidate for each pipeline and gives a link.

Alternatively, there could be a dedicated db table (superevent_pipelinepreferred or something like that) where each pipeline has its row with a foreign key entry that links to a pipeline's event. I think this is a valid approach that I can implement, as it would also return preferred events in igwn-alerts and API responses. The question there is, how does that table get populated?

For reference, there is no internal logic for defining a preferred event in GraceDB. GWCelery picks the preferred event.

1. Should the responsibility for defining a pipeline's preferred event be part of the pipeline? So at upload time, there could be an extra API call... client.set_as_pipeline_preferred(superevent_id, pipeline, event_id. The issue for doing this would be defining new groups and permissions. Or, if a user/robot has permission to populate a pipeline, then they can choose preferred events.
2. Have GWCelery (who is receiving alerts and filtering events already) by FAR/SNR and pipeline define it. This could be part of the superevent manager's logic. This kind of seems to be the natural choice to me.
3. Have GraceDB pick it. This requires more logic and selection criteria to be implemented and updated as the subject to the CBC and low-latency group's caprice. Then external groups need to define what the individual preferred selection criteria.

Personally I'm in favor of #2 (closed) (having the superevent manager choose the pipeline preferred event) as it is most parallel with what is being done currently.

Tagging some relevant parties:

@ian-harry, @erik-katsavounidis, @shaon.ghosh, @roberto.depietri