modqueue: fix performance regression from including appeals.

* Add index on posts.is_deleted. The modqueue was slow because we the
 appeal condition wasn't constrained to deleted posts, so it degraded to
 a full table scan.

* Avoid extra queries for calculating the page count and disapproval counts.

db/migrate/20200816175151_add_deleted_index_on_posts.rb

@@ -0,0 +1,5 @@
+class AddDeletedIndexOnPosts < ActiveRecord::Migration[6.0]
+  def change
+    add_index :posts, :is_deleted, where: "is_deleted = true"
+  end
+end