Discourse as a Mailing List improvements

FEATURE: context is not emailed if we previously emailed you the post
FEATURE: site setting to enable_watch_new_topics , false by default.
  When enables users can elect to watch everything by default
FIX: Custom email subjects (x quoted you in [title], x replied to [title])
  was removed, this broke email grouping. TBD, include info in footer somehow
FIX: topic user specs were messy, reduce side effects

lib/topic_creator.rb

@@ -32,6 +32,7 @@ class TopicCreator
     unless @opts[:auto_track] == false
       @topic.notifier.watch_topic!(@topic.user_id)
     end
+    TopicUser.auto_watch_new_topic(@topic.id)
   end
 
   def setup