FIX: Post uploads setting access_control_post_id unnecessarily (#26627)

This commit addresses an issue for sites where secure_uploads
is turned on after the site has been operating without it for
some time.

When uploads are linked when they are used inside a post,
we were setting the access_control_post_id unconditionally
if it was NULL to that post ID and secure_uploads was true.

However this causes issues if an upload has been used in a
few different places, especially if a post was previously
used in a PM and marked secure, so we end up with a case of
the upload using a public post for its access control, which
causes URLs to not use the /secure-uploads/ path in the post,
breaking things like image uploads.

We should only set the access_control_post_id if the post is the first time the
upload is referenced so it cannot hijack uploads from other places.

app/models/upload_reference.rb

@@ -6,6 +6,10 @@ class UploadReference < ActiveRecord::Base
 
   delegate :to_markdown, to: :upload
 
+  def target?(target_to_check)
+    self.target_id == target_to_check.id && self.target_type == target_to_check.class.to_s
+  end
+
   def self.ensure_exist!(upload_ids: [], target: nil, target_type: nil, target_id: nil)
     if !target && !(target_type && target_id)
       raise "target OR target_type and target_id are required"