Fix created external sky maps to have only non-negative values

There has been an example of a created external skymap that has negative values (M221770) and this resulted in a negative sky map overlap value and therefore a negative joint FAR. This should not be allowed as an event with a negative joint FAR would automatically pass publishing conditions.

This should be fixed by including changes in ligo-raven with ligo.raven.search.skymap_overlap and with the creation of external sky maps with gwcelery.tasks.external_skymaps.create_external_skymap by using abs().