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

spec/mailers/user_notifications_spec.rb

@@ -64,6 +64,9 @@ describe UserNotifications do
       # 1 unsubscribe
       mail.html_part.to_s.scan(/To unsubscribe/).count.should == 1
 
+      # side effect, topic user is updated with post number
+      tu = TopicUser.get(post.topic_id, response.user)
+      tu.last_emailed_post_number.should == response.post_number
     end
   end