title: How do I implement a maintenance mode / routing constraint?
body: <p>I want to implement a maintenance mode. I'm thinking of a check on all methods of all my controllers, each of which extend the <code>AbstractActionController</code> class.</p>
  
  <p>The goal is to simply check a boolean flag to see if maintenance mode is on and if so display a HTML page featuring a maintenance message (eg. We're currently performing maintenance. Please check back again in a few minutes)</p>
  
  <p><strong>Instead of manually performing the check in each function</strong>, how can I set up the check to be performed before any routing takes place?</p>