message: Fix sign up link
diff: {{#if (not session.isAuthenticated) }}
  <li class='sign-in'>
  <div>
  - <a href="{{_signupUrl}}" data-test-sign-up-button onclick={{action 'click' 'link' 'Navbar - SignUp' target=analytics}} class="btn btn-success btn-top-signup m-l-sm m-r-xs">
  + <a href="{{_signupURL}}" data-test-sign-up-button onclick={{action 'click' 'link' 'Navbar - SignUp' target=analytics}} class="btn btn-success btn-top-signup m-l-sm m-r-xs">
  {{t 'auth_dropdown.sign_up'}}
  </a>
  <a {{action 'login'}} data-test-sign-in-button onclick={{action 'click' 'button' 'Navbar - SignIn' target=analytics}} class="btn btn-info btn-top-login m-r-xs" role="button">
  