FEATURE: allow users to easily track/watch/mute topics via email

If you reply to an email with the word "mute" a topic will be muted
If you reply to an email with the word "track" a topic will be tracked
If you reply to an email with the word "watch" a topic will be watched

These ninja command can help advanced mailing list ex-users, saves a trip
to the website

lib/email/receiver.rb

@@ -899,6 +899,22 @@ module Email
       create_post_with_attachments(options)
     end
 
+    def notification_level_for(body)
+      # since we are stripping save all this work on long replies
+      return nil if body.length > 40
+
+      body = body.strip.downcase
+      case body
+      when "mute"
+        NotificationLevels.topic_levels[:muted]
+      when "track"
+        NotificationLevels.topic_levels[:tracking]
+      when "watch"
+        NotificationLevels.topic_levels[:watching]
+      else nil
+      end
+    end
+
     def create_reply(options = {})
       raise TopicNotFoundError if options[:topic].nil? || options[:topic].trashed?
       raise BouncedEmailError if options[:bounce] && options[:topic].archetype != Archetype.private_message
@@ -908,6 +924,8 @@ module Email
 
       if post_action_type = post_action_for(options[:raw])
         create_post_action(options[:user], options[:post], post_action_type)
+      elsif notification_level = notification_level_for(options[:raw])
+        TopicUser.change(options[:user].id, options[:post].topic_id, notification_level: notification_level)
       else
         raise TopicClosedError if options[:topic].closed?
         options[:topic_id] = options[:topic].id