Change pipeline preferred event in dashboard

This MR adds the ability to change pipeline preferred events from the dashboard. The dashboard will take 3 inputs: superevent id, pipeline, and new preferred event ID. It will make sure that the event ID provided is actually from the pipeline that the user input, and will check that the pipeline preferred event being changed is not currently the superevent's preferred event, and if it is it will not make the change. It also updates the "Change Preferred Event in Superevent" function to update the pipeline preferred event in the case where the new preferred event is not a pipeline preferred event. This last feature is still a WIP.

Closes #521 (closed).