PERF: Preload voters_count and has_voted (#28808)

These fields are often used when serializing topics which may contain
multiple polls. On average, serializing a poll took 2+N queries where N
is the number of options. This change reduces the number of queries to
3, one for each field (Poll#voters_count, PollOption#voters_count and
Poll#has_voted?).

plugins/poll/plugin.rb

@@ -182,12 +182,12 @@ after_initialize do
           end
 
         if post_with_polls.present?
-          Poll
-            .where(post_id: post_with_polls)
-            .each do |p|
-              polls[p.post_id] ||= []
-              polls[p.post_id] << p
-            end
+          all_polls = Poll.includes(:poll_options).where(post_id: post_with_polls)
+          Poll.preload!(all_polls, user_id: @user&.id)
+          all_polls.each do |p|
+            polls[p.post_id] ||= []
+            polls[p.post_id] << p
+          end
         end
 
         polls