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/views/conferences/_call_for_tracks.haml

@@ -1,4 +1,4 @@
-- cache [conference, conference.call_for_tracks, '#splash#callfortracks'] do
+- cache [conference_id, call, '#splash#callfortracks'] do
   .col-md-5.col-sm-5.text-center
     %h2
       Call for Tracks
@@ -8,13 +8,12 @@
       Would you like to host a mini-summit, sub-conference, or hack space?
       The submission period for track requests is open
       %em
-        = date_string(conference.call_for_tracks.start_date,
-          conference.call_for_tracks.end_date) + '.'
+        = date_string(call.start_date, call.end_date) + '.'
       %b
         You have
-        = pluralize(conference.call_for_tracks.remaining_days, 'day')
+        = pluralize(call.remaining_days, 'day')
         left!
     %p.cta-button
       = link_to("Submit your request for track",
-        conference_program_tracks_path(conference.short_title),
+        conference_program_tracks_path(conference_id),
         class: 'btn btn-success btn-lg text-center')