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/_highlights.haml

@@ -5,10 +5,10 @@
 .row
   .col-md-12
     .row.row-centered
-      - conference.highlighted_events.each do |event|
+      - highlights.each do |event|
         - speaker = event.speakers_ordered.first
         .col-md-3.col-sm-3.col-centered.col-top.highlights
-          = link_to(conference_program_proposal_path(conference.short_title,
+          = link_to(conference_program_proposal_path(conference_id,
             event),
             class: 'thumbnail') do
             = image_tag speaker.gravatar_url(size: 300),