title: django: give some users less permissions
body: <p>The app is installed at several customers and each installation has several groups and a lot of users.</p>
  
  <p>In the past every user was allowed to use the basic feature <em>BasicFeature</em>.</p>
  
  <p>A small new group ("RestrictedUsage") of users should not have the permission to have this <em>BasicFeature</em>.</p>
  
  <p>Our current automated update idea looks like this:</p>
  
  <ol>
  <li>Create new permission <em>UseBasicFeature</em></li>
  <li>Give all existing groups the new permission.</li>
  <li>Create the new group "RestrictedUsage"</li>
  </ol>
  
  <p>Since we have several customers, we need an automated way to handle this.</p>
  
  <p>The above solution does should work, but I don't like it.</p>
  
  <p>I don't like it because installations which don't have a "RestrictedUsage" group need to be modified. Otherwise the users would not be allowed to use <em>BasicFeature</em> (which they use since ages).</p>
  
  <p>Has anybody see better way?</p>