modqueue: optimize sql queries.

* Include appeals and flags.
* Avoid an existence query for pools.
* Avoid a query checking if the user has previously approved the post.
  This is a rare condition and it will be prevented anyway if the user
  tries to reapprove the post.

app/models/post_approval.rb

@@ -20,7 +20,7 @@ class PostApproval < ApplicationRecord
       errors.add(:base, "You cannot approve a post you uploaded")
     end
 
-    if post.approved_by?(user)
+    if post.approver == user || post.approvals.where(user: user).exists?
       errors.add(:base, "You have previously approved this post and cannot approve it again")
     end
   end