Add CSS validation to the testing pipeline

[Robbert]

• [x] Find CSS linting or validation package
• [x] Probably CSS validation is overkill, because we probably want to allow vendor prefixes. Instead, it would probably be a good idea to detect missing vendor prefixes for our target environments.

[Robbert]

The one written by Nicole Sullivan is csslint, so I would like to try that one.

I have defined our preferences in .csslintrcs.json. Thoughts on some of the settings:

• compatible-vendor-prefixes and fallback-colors: not wanted for development, but would be good for the dist/ directory
• known-properties: hopefully they'll stay up to date, or else I won't be able to use the very latest properties without breaking the build.
• shorthand: I think this an optimization best left to a CSS pre-processor.

Not sure yet about: overqualified-elements, universal-selector, import, box-sizing.