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

app/services/user_updater.rb

@@ -30,7 +30,8 @@ class UserUpdater
       :email_private_messages,
       :external_links_in_new_tab,
       :enable_quoting,
-      :dynamic_favicon
+      :dynamic_favicon,
+      :watch_new_topics
     ].each do |attribute|
       if attributes[attribute].present?
         user.send("#{attribute.to_s}=", attributes[attribute] == 'true')