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/_venue_map.haml

@@ -1,13 +1,13 @@
 #map{ style: "height: 500px;" }
 - content_for(:script_body) do
   - marker = escape_javascript(render '/conferences/venue_map_marker',
-    venue: conference.venue)
+    venue: venue)
   :javascript
     // create a map in the "map" div, set the view to a given place and zoom
     var map = L.map('map', { scrollWheelZoom: false }).setView(
       [
-        #{conference.venue.latitude.to_f + 0.05},
-        #{h(conference.venue.longitude)}
+        #{venue.latitude.to_f + 0.05},
+        #{h(venue.longitude)}
       ], 11);
     // add an OpenStreetMap tile layer
     L.tileLayer('//{s}.tile.openstreetmap.org/{z}/{x}/{y}.png', {
@@ -20,8 +20,8 @@
     // add a marker in the given location, attach some popup content to it
     // and open the popup
     L.marker([
-      #{h conference.venue.latitude},
-      #{h conference.venue.longitude}
+      #{h venue.latitude},
+      #{h venue.longitude}
     ]).addTo(map)
       .bindPopup("#{marker}")
       .openPopup();