Style the new Thrasher container

Now that @robertberry has added a `.fc-container--thrasher` class, we can move a lot of the repeating code added to every thrasher and move it into frontend. Basically it's a slimmed down version of [this mixin](https://github.com/guardian/thrashers/blob/master/shared/_mixins.scss#L3).

A few properties will still have to be declared in the thrasher itself, but this massively reduces repeating code and keeps 99% of the global properties next to the originals.

![200](https://cloud.githubusercontent.com/assets/1607666/5339199/419fbe08-7ed5-11e4-89c9-ba52972f45bf.gif)

Use $gs-gutter