Create and process external MDC events to test RAVEN pipeline; fixes #401 and #403

These changes create and upload external MDC events in order to regularly test the fully RAVEN pipeline. This is designed to create an external MDC event for every 2 MDC superevents, resetting every day. These joint coincidences should be able to be found by ligo-raven>=2.0 by including search='MDC' in ligo.raven.search.search().

The MR includes changes to:

• create MDC external events via first2years_external.py
• use se_search field for superevent searches with new release of ligo-raven
• update superevent with coinc FAR values when better coincidence is available

gwcelery/tasks/raven.py

@@ -187,19 +187,19 @@ def raven_pipeline(raven_search_results, gracedb_id, alert_object, tl, th,
     """
     if not raven_search_results:
         return
-    if alert_object.get('group') == 'External':
+    if 'S' not in gracedb_id:
         raven_search_results = preferred_superevent(raven_search_results)
     for result in raven_search_results:
-        if alert_object.get('group') == 'External':
-            superevent_id = result['superevent_id']
-            exttrig_id = gracedb_id
-            superevent = result
-            ext_event = alert_object
-        elif 'S' in gracedb_id:
+        if 'S' in gracedb_id:
             superevent_id = gracedb_id
             exttrig_id = result['graceid']
             superevent = alert_object
             ext_event = result
+        else:
+            superevent_id = result['superevent_id']
+            exttrig_id = gracedb_id
+            superevent = result
+            ext_event = alert_object
 
         canvas = (
             gracedb.add_event_to_superevent.si(superevent_id, exttrig_id)