/moderator/dashboard: optimize sql queries

* Converts queries to use active record instead of raw sql. This ensures
  that user objects are loaded by rails in the join, so that we
  don't have to issue `User.find` calls to load users one-by-one.

* Use `.includes` to preload associations used in the view, to avoid
  additional N+1 query problems (primarily, calls to link_to_user
  also causing users to be loaded one-by-one).

app/logical/moderator/dashboard/queries/upload.rb

@@ -1,28 +1,16 @@
 module Moderator
   module Dashboard
     module Queries
-      class Upload
-        attr_reader :user, :count
-
+      class Upload < ::Struct.new(:user, :count)
         def self.all(min_date, max_level)
-          sql = <<-EOS
-            select uploader_id, count(*)
-            from posts
-            join users on uploader_id = users.id
-            where
-              posts.created_at > ?
-              and level <= ?
-            group by posts.uploader_id
-            order by count(*) desc
-            limit 10
-          EOS
-
-          ActiveRecord::Base.select_all_sql(sql, min_date, max_level).map {|x| new(x)}
-        end
-
-        def initialize(hash)
-          @user = ::User.find(hash["uploader_id"])
-          @count = hash["count"]
+          ::Post.joins(:uploader)
+            .where("posts.created_at > ?", min_date)
+            .where("users.level <= ?", max_level)
+            .group(:uploader)
+            .order("count(*) desc")
+            .limit(10)
+            .count
+            .map { |user, count| new(user, count) }
         end
       end
     end