Add ability to load GW sky map from preferred event

There has been need to add the ability to load the GW sky map from the preferred event due to the issue emfollow/gwcelery#547 (closed) .

This lets us avoid this issue of the sky map not being available in certain superevents by having the option to grab it from the preferred event instead, where we will ensure this is present by the presence of the EM_READY label.