Rework create_upload_external_event to use multiple searches at once

Currently first2years_external.create_upload_external_event can only pick a single searches (GRB, SubGRB, SubGRBTargeted) and populates pipelines/events based on that. A better design would be to select searches independently (meaning returning a list of searchs ['GRB', 'SubGRBTargeted', 'GRB'] including possible repeats) and then picking the pipeline(s)/event(s) from that.

This design will let us better test instances of multiple searches, especially how the system will choose which is the preferred external event based on the search field. This setup is also more realistic.