If sky map not available yet in superevent, use sky map in preferred event for RAVEN sky map comparison; fixes #547

This allows the RAVEN comparison workflow to work for events with FAR<2/day that will never have a sky map copied to the superevent by grabbing the sky map from the preferred event via presence of the EM_READY label (representing that the preferred event is complete and therefore has a sky map).

Our current design for sky map comparison is that IGWN alert listener is looking at each external events whether to launch the sky map comparison (some external events may get their own sky maps quicker or not at all and should be calculated separately). If see see EM_READY, we will apply this label to all related external events so they know the preferred sky map is available, similarly to how we use SKYMAP_READY to know that the superevent sky map is available.

Note that we will default back to grabbing sky maps from the superevent if the standard SKYMAP_READY label is applied to the superevent. Also grabbing from the preferred event initially should remove that largest source of latency for the sky map workflow (waiting for the sky map to be available in the superevent).