FIX: Notify on Reviewable update. (#7980)

If a post is flagged after an action was already performed on it, it
will update the previous Reviable instance and not create a new one.
The notification logic was implemented in the :create callback which was
completely skipped in this case.

app/models/reviewable.rb

@@ -38,6 +38,9 @@ class Reviewable < ActiveRecord::Base
 
   after_commit(on: :create) do
     DiscourseEvent.trigger(:reviewable_created, self)
+  end
+
+  after_commit(on: [:create, :update]) do
     Jobs.enqueue(:notify_reviewable, reviewable_id: self.id) if pending?
   end