add get_retina_url helper method

So you can get the retina url for css background images.

ex:
.school-hero-unit {
    background-image: url('#{image.url(:banner)}');
  }

  @media
  (-webkit-min-device-pixel-ratio: 2),
  (min-resolution: 192dpi) {
    .school-hero-unit-bg {
      background-image: url('#{get_retina_url(image.url(:banner))}');
    }
  }