Hide advert in lightbox mode

### Before

![screen shot 2014-05-27 at 14 13 49](https://cloud.githubusercontent.com/assets/74132/3091968/bf077ffa-e5a3-11e3-98d5-083ae462c6f1.png)

### After

![screen shot 2014-05-27 at 14 13 45](https://cloud.githubusercontent.com/assets/74132/3091970/c3053bec-e5a3-11e3-81e5-2e5c63379f62.png)

NB: Also had to fix a sass lint issue - looks like the build run `validate:sass`, going to fix