Override column-fill property on popular container at tablet breakpoint. Fixes: #6700

This fixes #6700 turns out both Safari and Firefox were behaving correctly, as the [column-fill](https://developer.mozilla.org/en-US/docs/Web/CSS/column-fill) property defaults to `balance`

This is a temporary fix for US launch until I can submit a PR to guss-layout to enable the author to pass in a column-fill parameter to the column mixin.

/cc @michaelwmcnamara @ironsidevsquincy @sndrs 

### Before
![safari](https://cloud.githubusercontent.com/assets/80089/4813449/93177d48-5ec8-11e4-9ffa-ea39e08439a4.png)

### After
![safari](https://cloud.githubusercontent.com/assets/80089/4813450/972dec8c-5ec8-11e4-9d3f-5e1f2d985b50.png)