Update confirm_superevent_as_gw utility method

Added some information about old/new IDs to the log message which
is created when a superevent is "upgraded" to a GW.

gracedb/superevents/utils.py

@@ -480,18 +480,20 @@ def get_superevent_by_date_id_or_404(superevent_id, queryset=None):
 def confirm_superevent_as_gw(superevent, user, add_log_message=True,
     issue_alert=True):
 
+    # Save old ID temporarily
+    old_id = superevent.superevent_id
+
     # Update superevent (mark as a GW, construct new ID, etc.)
     superevent.confirm_as_gw()
 
     # Create log message
     gw_log = None
     if add_log_message:
-        message = "Confirmed as a gravitational wave."
+        message = ("Confirmed as a gravitational wave: ID changed from "
+            "{old} -> {new}").format(old=old_id, new=superevent.superevent_id)
         gw_log = create_log(user, message, superevent, issue_alert=issue_alert,
             autogenerated=False)
 
-    # TODO: add label?
-
     return gw_log