FEATURE: Allow admin to auto reopen at topic.

* This commit also introduces a `TopicStatusUpdate`
  model to support other forms of deferred topic
  status update in the future.

spec/fabricators/topic_status_update_fabricator.rb

@@ -0,0 +1,6 @@
+Fabricator(:topic_status_update) do
+  user
+  topic
+  execute_at Time.zone.now + 1.hour
+  status_type TopicStatusUpdate.types[:close]
+end