Fix code standards

I realise some of these may seem personal, but the suggested changes bring the SCSS code standards more into line with the CSS code standards.

Here's the `.scss-lint.yml` file I'm using: https://github.com/GaryJones/incipio/blob/develop/.scss-lint.yml - this isn't supported officially by WP, but I think it matches the WP CS for CSS as close as possible, and I wouldn't expect any official standard to deviate to far from this in the long-term.

I'll leave comments within the PR for any other comments.
The `url()` example on the WPCS for CSS page doesn't show a space, but I could well imagine that any SCSS standard would allow for spaces in mixin calls, to match PHP and JS standards.