Make Pool#posts return an array instead of arel

fixes #2042

app/models/pool.rb

@@ -241,9 +241,11 @@ class Pool < ActiveRecord::Base
     limit = options[:limit] || Danbooru.config.posts_per_page
     slice = post_id_array.slice(offset, limit)
     if slice && slice.any?
-      Post.where("id in (?)", slice).order(arbitrary_sql_order_clause(slice, "posts"))
+      slice.map do |id|
+        Post.find(id)
+      end
     else
-      Post.where("false")
+      []
     end
   end