Homepage adjustments from creative + other goodies

  + add new banner images (awf, ch, erd)
  + create reusable class for banner bg images and use
    that class rather than declaring the same bg image in
    several different places
  + add styles defining a container with two different
    background colors for those pesky banners/slides that
    have a different color on either side
  + reorganize code to work with new banner classes
  + optimize images
Merge branch 'homepage_adjustments' into develop