FIX: Posts can belong to hard-deleted topics (#17329)

* FIX: Posts can belong to hard-deleted topics

This was a problem when serializing deleted posts because they might
belong to a topic that was permanently deleted. This caused to DB
lookup to fail immediately and raise an exception. In this case, the
endpoint returned a 404.

* FIX: Remove N+1 queries

Deleted topics were not loaded because of the default scope that
filters out all deleted topics. It executed a query for each deleted
topic.

app/controllers/posts_controller.rb

@@ -705,10 +705,13 @@ class PostsController < ApplicationController
   private
 
   def user_posts(guardian, user_id, opts)
-    posts = Post.includes(:user, :topic, :deleted_by, :user_actions)
-      .where(user_id: user_id)
-      .with_deleted
-      .order(created_at: :desc)
+    # Topic.unscoped is necessary to remove the default deleted_at: nil scope
+    posts = Topic.unscoped do
+      Post.includes(:user, :topic, :deleted_by, :user_actions)
+        .where(user_id: user_id)
+        .with_deleted
+        .order(created_at: :desc)
+    end
 
     if guardian.user.moderator?