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/_tickets.haml

@@ -2,7 +2,7 @@
   %li
     %a.smoothscroll{ href: '#tickets' } Tickets
 
-- cache [conference, conference.tickets, '#splash#tickets'] do
+- cache [conference, tickets, '#splash#tickets'] do
   %section#tickets
     .container
       .row
@@ -11,7 +11,7 @@
             Support
             = conference.title
       .row.row-centered
-        - conference.tickets.each do |ticket|
+        - tickets.each do |ticket|
           .col-md-3.col-sm-3.col-centered.col-top
             = link_to(conference_tickets_path(conference.short_title),
               class: 'thumbnail') do