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.
Clear line spaces at the end of files is part of the PHP standard, and general code practice for all files. End of line characters are not the same as End of file characters, so future diffs become cleaner.