CSS Audit

[talldan]

Closes #80

• Adds stylelintrc - the one from Gutenberg
• Adds sass package (as an alias to node-sass)
• Remove unused styles (in app/style.css)
• Switch to BEM style naming convention block__element--modifier and take advantage of sass interpolation.
• Move styles from app to pages (header, title, etc.)
• Move styles from tabs to preferences (input, directory-select, etc.)

[talldan]

Ready for review.

[pento]

NPM 6.9 allows us to alias sass to node-sass, by installing it as npm install node-sass@npm:sass (ref).

[talldan]

@pento 👍 switched to sass as recommended.

[pento]

Looks good! Let's get this in, so we can rebase all of the PRs waiting for it. 🙂