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/proposals/_encouragement_text.html.haml

@@ -1,7 +1,7 @@
 %p.lead
   - if @program.event_types.any?
     You can submit proposals for
-    = "#{event_types(@conference)}."
+    = "#{event_types_sentence(@conference)}."
   - if @program.tracks.confirmed.cfp_active.any?
     Proposals should fit in one of the
     = "#{pluralize(@program.tracks.confirmed.cfp_active.count, 'track')}:"