Break up the one-query-of-doom

The single query was producing a tremendously large ActiveRecord allocation.

While this isn't nearly as cool as one query, it's still a significant
reduction in SQL trips, and now has the bonus of only loading what is
actually going to be displayed.

app/helpers/conference_helper.rb

@@ -1,8 +1,7 @@
 module ConferenceHelper
   # Return true if only call_for_papers or call_for_tracks is open
-  def one_call_open(conference)
-    conference.call_for_events.try(:open?) ^
-      conference.call_for_tracks.try(:open?)
+  def one_call_open(*calls)
+    calls.one? { |call| call.try(:open?) }
   end
 
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