UX: remove gray background from lighbox

This avoids a 3 way transition when loading images

- grey
- ultra low res
- full image

Instead we go

- ultra low res
- full image

app/assets/stylesheets/common/base/lightbox.scss

@@ -2,7 +2,6 @@
   position: relative;
   display: inline-block;
   overflow: hidden;
-  background: $primary-low;
   &:hover .meta {
     opacity: 0.9;
     transition: opacity 0.5s;