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
was fixed in af97affba666d0d6194214c2477f64ca7ba3c847, which I believe
demonstrates 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.