Rework RAVEN pipeline canvas

There at least two issues that are present in the current iteration of the raven.raven_pipeline canvas:

• ❌ EM_COINC should be applied before checking publishing conditions to prevent race condition of RAVEN_ALERT being applied before EM_COINC
• We should not plot the overlap integral with external_skymaps.plot_overlap_integral for other external events once RAVEN_ALERT has been applied, i.e. only plot skymap overlap for preferred external event once RAVEN_ALERT applied to superevent

This changed design could look like:

            |
            update_coinc_far.s(superevent, ext_event)
            |
            group(gracedb.create_label.si('EM_COINC', superevent_id),
                  gracedb.create_label.si('EM_COINC', exttrig_id))
        )

        canvas_final = trigger_raven_alert.s(superevent, gracedb_id,
                                             ext_event, gw_group),
        if superevent['em_type']==ext_event['graceid'] or 'RAVEN_ALERT' not in superevent['labels']:
            canvas_final += external_skymaps.plot_overlap_integral.s(
                                superevent_id, exttrig_id),
        canvas |= group(canvas_final)

        canvas.delay()