Add scss-lint

Please say if you think any of the linters that are enabled are going too far. My intention is to introduce this in a way that maintains the consistency of our Sass without being too demanding.

I've made a change to `_grid_layout.scss` and I don't particularly understand how `%outdent-to-full-width` is meant to work, so take a look.

This only affects the way Travis runs tests at the moment.

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*Commit message:*

Lint our Sass files to allow us to accept patches with increased confidence that they are free of bugs and will fit in with our existing Sass well.

I've tried to enable only the linters that I think will be relatively uncontroversial. My intention was to start off basic (eg ensuring that indentation is consistent across the toolkit).

Enabling linting has noticed a problem in `_grid_layout.scss` which I believe is a bug, which I think demonstrates that this is something worth introducing.

I've also made some changes to `_media-player.scss` which might make the styles easier to read without changing the behaviour.