Add SCSS Linter

Added SCSS Linting

Status: **Ready to merge**
Reviewers: @ry5n @jeffkamo @scalvert @stewartyu @MikeKlemarewski 

## Changes

- Updates the preferred order for authoring CSS rules;
- Adds missing properties (e.g. latest flexbox spec, some other obscure ones);
- Removes any vendor-prefixed properties that have a standards version (since autoprefixer will take care of that);
- Adds a `.scss-lint.yml` config file for use with the scss-lint tool.

## Notes
- To test this out:
    - add `.scss-lint.yml` to your project root
    - Install the `Sublime Linter` plugin in Sublime
    - Install the SCSS Lint plugin for Sublime Linter (instructions here: https://sublime.wbond.net/packages/SublimeLinter-contrib-scss-lint)
    - If you're not using Sublime, you can just use the SCSS Lint gem instead.