Add stylelint config and make less compliant with CS (part 1)

Stage one of adding stylelint, is getting the basics in place and keeping our less files linted

1. Add stylelint config matching css coding style
2. Add stylelint checking to grunt
3. Ensure less files pass lint before building
4. Fix all the stylistic lint errors in less (which don't impact on actual built css, the ones which affect the css will be fixed in MDL-55165)