Add scss-lint

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.