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/_booths.haml

@@ -2,7 +2,7 @@
   .col-md-12
     %h3.text-center Booths
 .row.row-centered
-  - conference.confirmed_booths.to_a.sort_by!(&:title).each do |booth|
+  - booths.each do |booth|
     .col-md-2.col-sm-2.col-centered.col-top.booth
       %a.thumbnail{ href: '#',
         data: { toggle: 'modal', target: "#modal-booth-#{booth.id}" } }