Add a desktop override mixin

On level 1 sites we often need to style on top of some truly gnarly existing CSS. This mixin helps overcome extreme speficicity while otherwise allowing us to maintain our normal CSS standards. It also avoids the temptation to use !important for this purpose, keeping it in reserve for when it’s really necessary.

Status: **Opened for visibility**

Reviewers: **@kpeatt @jeffkamo @tedtate **

## Changes
- Adds a new (re-written) version of the experimental desktop-override mixin. This version was developed after running into limitations with the old version on an actual client project.
- This version is simpler (relying on the ability to chain the same id selector to itself as many times as needed).
- It also works in more scenarios, being able to style any element within `<body>`, not just within `<div id="x-root">`.
- The only requirement is a consistent id attribute on the `<head>` tag across all pages.

## How to test-drive this PR
- TBD