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/_registration.haml

@@ -2,8 +2,7 @@
   %li
     %a.smoothscroll{ href: '#registration' } Registration
 
-- cache [conference.registration_period, conference.tickets,
-  '#splash#registration'] do
+- cache [conference, registration_period, tickets, '#splash#registration'] do
   %section#registration
     .container
       .row
@@ -12,23 +11,23 @@
           - if conference.registration_limit_exceeded?
             %p
               Sorry, the conference registration limit has exceeded
-          - elsif conference.tickets.empty?
+          - elsif tickets.empty?
             %p.lead
               Going to
               = conference.short_title
               is free of charge.
             %p
               We only ask you to register yourself before
-              = date_string(conference.registration_period.end_date)
+              = date_string(registration_period.end_date)
               so we can plan for the right amount of people.
               %p.cta-button
           - else
             %p
               The registration period is open
-              = date_string(conference.registration_period.start_date,
-                conference.registration_period.end_date)
+              = date_string(registration_period.start_date,
+                registration_period.end_date)
           - if conference.registration_open?
             %p.cta-button
               = link_to('Register Now',
-                new_conference_conference_registration_path(conference.short_title),
+                new_conference_conference_registration_path(conference_id),
                 class: 'btn btn-lg btn-success')