Move `raven.update_coinc_far` and `external_skymaps.plot_overlap_integral` into `raven.trigger_raven_alert`

Currently both raven.update_coinc_far and external_skymaps.plot_overlap_integral and depend on whether the RAVEN_ALERT label is applied, something done later in the canvas by raven.trigger_raven_alert. Instead if the joint candidates pass the publishing threshold, then we add this label to the superevent dictionary and then run these two functions within the already existing canvas in raven.trigger_raven_alert.