Do not display collection icons if a background image is included (bug 1049193)

r? @ngokevin @spasovski

Note: this does not fix the existing bug where a promo does not expand past its min-height because its contents are all absolutely positioned.

## Before

![screen shot 2014-08-07 at 10 47 23 am](https://cloud.githubusercontent.com/assets/23885/3845469/d852ad44-1e52-11e4-88f9-b58a89ad669c.png)

## After

![screen shot 2014-08-07 at 10 47 28 am](https://cloud.githubusercontent.com/assets/23885/3845471/dd224f6e-1e52-11e4-84ae-3198b9664f5d.png)
This is less readable but a neat approach. Still think I'd prefer it nested under `&.has-background` but not enough to r-. Your call.