Replacing a potentially expensive query (#17605)

Prior to this commit, if we didn't have an instance of `Article` we
likely had the class `Article`; we would then run a potentially VERY
expensive query (`SELECT user_id FROM articles;`) to provide an
answer; namely if all of the articles are written by the same user then
yes this user is the author.

With this commit, when you're authorizing the `Article` class, let's
just say "Nope, this user isn't the author".

Closes forem/forem#17604

app/policies/article_policy.rb

@@ -200,11 +200,10 @@ class ArticlePolicy < ApplicationPolicy
   private
 
   def user_author?
-    if record.instance_of?(Article)
-      record.user_id == user.id
-    else
-      record.pluck(:user_id).uniq == [user.id]
-    end
+    # We might have the Article class (instead of the Article instance), so let's short circuit
+    return false unless record.respond_to?(:user_id)
+
+    record.user_id == user.id
   end
 
   def user_org_admin?