Improve colour management in stylesheets

Add a README that contains colour swatches.

Use Sass variables to reference colours. Ideally we'd get
rid of the mixing I've added and just use standard greys,
but for now it's still nicer to use proper variables and
not just magic hex values.

Remove unused $primary-colour-text variable.

There shouldn't be any noticeable difference in the browser
as a result of this commit.
Move direct styles before extensions

When using an extension (ampersand), you generally want the styles
that apply to the element to be overridden by the thing you're
extending them with.

Use the cascade properly by applying the extended properties after
the properties that apply directly to the element you're targeting.
Use box-sizing mixin from frontend toolkit

The frontend toolkit provides this code, so we shouldn't
duplicate it here.
Switch README badges to SVG for hi-res funtimes
Add Sass comments, switch scaleX to scale

By using scale rather than scaleX we can take advantage of a
mixin in the frontend toolkit.