Improve the YouTube consent placeholder

The YouTube consent placeholder is currently a shortcode, which could be nice but the markdown parser gets in the way.

A better way to do it would be to define a `youtube-player` web component that asks for consent. The component would be defined in a `/js/components.js` file in which all the web components used on m.org are defined (so far only the youtube-player). That file would need to be added globally via the `skel.html` template. This allows us to use `` everywhere without having to worry about embedding the `components.js` file

It should also be possible to un-consent, likely by going to the privacy policy page and adding a button to set the `ytConsent` key to `false` in the localStorage

Start by reviewing the existing YouTube shortcode and the global skel.html template, then inspect the privacy policy page for the consent controls. Define the youtube-player web component in /js/components.js, make it available globally, and ensure the privacy page can unset ytConsent; done means the component can be used throughout the site without markdown-parser issues and consent can be withdrawn.

Written by the indexing model from the issue text.