Create a shared set of mixins bespoke to the thrashers.

Create shared set of mixins for all thrashers. So far `shared/_mixins.scss` contains two mixins.

- `full-width($containerName, $c-background: #dedede);` This makes containers full-width by overwriting existing styling.
- `rule($containerName, $c-rule: #005689);` This adds a border to the top of the thrasher. Needs to be a mixin as it too overwrites existing styling.

I imagine this partial will expand as patterns emerge from the actual `embeds`.

The only files changed here is `shared/_mixins.scss` the rest will be handled when [this pull request is merged in](https://github.com/guardian/thrashers/pull/8).

ABSTRACTION!

![giphy](https://cloud.githubusercontent.com/assets/1607666/5089547/0fbfef1c-6f34-11e4-8d5d-7d89ec28a7dc.gif)