Remake combined sky map every new GW or GRB sky map

Currently we only create a combined sky map once when we send out a joint alert, but we should also remake this combined sky map whenever another GW sky map supercedes this (say from a new preferred event or a sky map from bilby) or there is a new GRB sky map (say we copy from a URL or download from HEASARC).

When sending alerts we should then copy the latest combined sky map from the external event.