posts: allow admins to approve the same post twice.

This is so admins can overrule flags and always have the final say in whether a
post is approved, even in the event of coordinated or sockpuppet flagging.

Fixes #4980: Way to mark flags as invalid for admins

app/models/post_approval.rb

@@ -18,7 +18,7 @@ class PostApproval < ApplicationRecord
       errors.add(:base, "You cannot approve a post you uploaded")
     end
 
-    if post.approver == user || post.approvals.exists?(user: user)
+    if (post.approver == user || post.approvals.exists?(user: user)) && !policy(user).can_bypass_approval_limits?
       errors.add(:base, "You have previously approved this post and cannot approve it again")
     end
   end

app/policies/post_approval_policy.rb

@@ -4,4 +4,8 @@ class PostApprovalPolicy < ApplicationPolicy
   def create?
     user.is_approver?
   end
+
+  def can_bypass_approval_limits?
+    user.is_admin?
+  end
 end