title: Authenticate user with a URL token in an MVC web application
body: <p>I would like to add a user authentication flow to an exiting MVC 4 web application, where the user will click on a link in an email that includes a URL token. This should take him to the application and automatically log him in. 
  This token could be added to any existing request to allow transparent login.</p>
  
  <p>What would be the right place in the framework to add the check for the token?</p>
  
  <p>Please. <strong>I am aware of the security risks</strong> of this feature. So please don't analyze the security aspects of this.</p>
  
  <p>I <strong>would</strong> like to hear about the possible implementations of this feature</p>