FIX: Keep ReviewableQueuedPosts even with user delete reviewable actions (#22501)

Performing a `Delete User`/`Delete and Block User` reviewable actions for a
queued post reviewable from the `review.show` route results in an error
popup even if the action completes successfully.

This happens because unlike other reviewable types, a user delete action
on a queued post reviewable results in the deletion of the reviewable
itself. A subsequent attempt to reload the reviewable record results in
404. The deletion happens as part of the call to `UserDestroyer` which
includes a step for destroying reviewables created by the user being
destroyed. At the root of this is the creator of the queued post
being set as the creator of the reviewable as instead of the system
user.

This change assigns the creator of the reviewable to the system user and
uses the more approapriate `target_created_by` column for the creator of the
post being queued.

app/models/reviewable_queued_post.rb

@@ -16,6 +16,12 @@ class ReviewableQueuedPost < Reviewable
 
   after_commit :compute_user_stats, only: %i[create update]
 
+  def updatable_reviewable_scores
+    # Approvals are possible for already rejected queued posts. We need the
+    # scores to be updated when this happens.
+    reviewable_scores.pending.or(reviewable_scores.disagreed)
+  end
+
   def build_actions(actions, guardian, args)
     unless approved?
       if topic&.closed?
@@ -25,9 +31,11 @@ class ReviewableQueuedPost < Reviewable
           a.confirm_message = "reviewables.actions.approve_post.confirm_closed"
         end
       else
-        actions.add(:approve_post) do |a|
-          a.icon = "check"
-          a.label = "reviewables.actions.approve_post.title"
+        if target_created_by.present?
+          actions.add(:approve_post) do |a|
+            a.icon = "check"
+            a.label = "reviewables.actions.approve_post.title"
+          end
         end
       end
     end
@@ -39,7 +47,7 @@ class ReviewableQueuedPost < Reviewable
       end
     end
 
-    delete_user_actions(actions) if pending? && guardian.can_delete_user?(created_by)
+    delete_user_actions(actions) if pending? && guardian.can_delete_user?(target_created_by)
 
     actions.add(:delete) if guardian.can_delete?(self)
   end
@@ -79,7 +87,7 @@ class ReviewableQueuedPost < Reviewable
       )
     opts.merge!(guardian: Guardian.new(performed_by)) if performed_by.staff?
 
-    creator = PostCreator.new(created_by, opts)
+    creator = PostCreator.new(target_created_by, opts)
     created_post = creator.create
 
     unless created_post && creator.errors.blank?
@@ -90,7 +98,7 @@ class ReviewableQueuedPost < Reviewable
     self.topic_id = created_post.topic_id if topic_id.nil?
     save
 
-    UserSilencer.unsilence(created_by, performed_by) if created_by.silenced?
+    UserSilencer.unsilence(target_created_by, performed_by) if target_created_by.silenced?
 
     StaffActionLogger.new(performed_by).log_post_approved(created_post) if performed_by.staff?
 
@@ -99,7 +107,7 @@ class ReviewableQueuedPost < Reviewable
 
     Notification.create!(
       notification_type: Notification.types[:post_approved],
-      user_id: created_by.id,
+      user_id: target_created_by.id,
       data: { post_url: created_post.url }.to_json,
       topic_id: created_post.topic_id,
       post_number: created_post.post_number,
@@ -148,9 +156,12 @@ class ReviewableQueuedPost < Reviewable
   private
 
   def delete_user(performed_by, delete_options)
-    reviewable_ids = Reviewable.where(created_by: created_by).pluck(:id)
-    UserDestroyer.new(performed_by).destroy(created_by, delete_options)
-    create_result(:success) { |r| r.remove_reviewable_ids = reviewable_ids }
+    reviewable_ids = Reviewable.where(created_by: target_created_by).pluck(:id)
+
+    UserDestroyer.new(performed_by).destroy(target_created_by, delete_options)
+    update_column(:target_created_by_id, nil)
+
+    create_result(:success, :rejected) { |r| r.remove_reviewable_ids += reviewable_ids }
   end
 
   def delete_opts
@@ -164,7 +175,7 @@ class ReviewableQueuedPost < Reviewable
 
   def compute_user_stats
     return unless status_changed_from_or_to_pending?
-    created_by.user_stat.update_pending_posts
+    target_created_by&.user_stat&.update_pending_posts
   end
 
   def status_changed_from_or_to_pending?