codepen: demos are not scrollable

[devversion]

At the moment there are situations when scrolling in a codepen demo isn't possible.

• Codepen demos from HEAD are not affected, because the demo.cssscript is not injected probably.

• Relases are affected, because the demo.css is referenced / injected correctly.

  Little example: https://material.angularjs.org/latest/demo/card

---

For example in HEAD builds the docs.css will be injected using the current commit SHA and not the latest release tag. (Like so - https://material.angularjs.org/master-9e7937e/docs.css)

So the actual problem on releases' demo is:

body {
  overflow: hidden;
  max-width: 100%;
  max-height: 100%;
}

I'm not suggesting to remove that overflow property or neither stopping injecting the docs.css.
A really elegant solution will be, to add a unique identifier to the CSS selector which will be only used in the docs, not in the demos.

Includes - #7216