title: Wagtail PASSWORD_REQUIRED_TEMPLATE is not overriding the default login
body: <p>I'm building a global login page for Wagtail. The setting for PASSWORD_REQUIRED_TEMPLATE isn't working. In fact, i can't find n example where it actually does work which makes me think that I don't understand what it's supposed to do.</p>
  
  <p>When I add; <code>PASSWORD_REQUIRED_TEMPLATE = 'utils/auth/password-required.html'</code> to the settings file it does not catch the login form and use my custom form.</p>
  
  <p>Is this a know issue?</p>