More advance unsubscribing via email

( As discussed and specified here: https://meta.discourse.org/t/notification-subscription-options/13328/8 )
 - emails now always contain the reply-key
 - which is used to compile direct links in the footer to change the notification level
 - the controller behind it changes the notification level for the topic and user giving through the reply-key without any other identification for users convenience
 - it spits out an immediate no_js-layouted website showing the topic and the new notification level with a handy link to the topic for quick feedback on click even on mobile
 - contains test (which on current master seem to not be run for other reasons)

This is what the simple confirmation then looks like:
![screen shot 2014-03-28 at 12 04 25](https://cloud.githubusercontent.com/assets/40496/2548794/c856159a-b668-11e3-8c1e-2572558c2a2b.png)