Styles: use mixin for Image styles in Image and RichText component

[justintemps]

Right now, we're basically copying the Image styles in the RichText component to style its images.

You can see this done here: a9a1b77

This is not a good solution, instead we should use a mixin and import it in both places, the way we do with _breadcrumb.scss in _richtext.scss. That way, the styled will be kept in sync in both places.