FEATURE: make pin expiration mandatory

app/serializers/topic_view_serializer.rb

@@ -43,6 +43,7 @@ class TopicViewSerializer < ApplicationSerializer
              :pinned_globally,
              :pinned,    # Is topic pinned and viewer hasn't cleared the pin?
              :pinned_at, # Ignores clear pin
+             :pinned_until,
              :details,
              :highest_post_number,
              :last_read_post_number,
@@ -177,6 +178,10 @@ class TopicViewSerializer < ApplicationSerializer
     object.topic.pinned_at
   end
 
+  def pinned_until
+    object.topic.pinned_until
+  end
+
   def actions_summary
     result = []
     return [] unless post = object.posts.try(:first)