Move some creation of combined sky map workflow into orchestrator.py

Currently combined sky maps always contain GW sky maps from the previous alert (not the alert they are associate with) since we don't start creating the combined sky map until the GW sky map is added to the superevent during creating alert, almost never finishing in time to then download the latest combined sky map. This could be fixed by creating the combined sky map when issuing the alert at the same time we usually download the GW sky map. This would also mean we would turn off the automatic creation of combined sky maps until after ADVOK is applied (i.e. add a check for ADVOK here).

A complication of this would be adding latency, which could be more pronounced if we give support to multiordered GRB sky maps with #581 (closed). Also note that much of workflow here in orchestrator.py would need to be moved and we may lose references to the latest version in the .png message.