demo(youtube-player): add auto video resizing

[Splaktar]

• automatically resize the video to fit the screen
  • this was noticeable on mobile where the video would just get cut off
• rename video to selectedVideo for clarity
• fix TypeScript path mappings so that IDEs don't complain about module
• use classes to style Material components as recommended

[YourDeveloperFriend]

I wonder if this is highlighting a bigger problem with the component: most users would expect the youtube video to fill the width of the youtube-player element. Rather than forcing the user to explicitly set a width and height, I wonder if it should default to whatever the css style of the youtube-player element.

[Splaktar]

@YourDeveloperFriend I do think that there is a case for a fixed size API, but I agree that there also should be some kind of autoSize mode that fits the video automatically. It's not something that is trivial for many developers to do.

[jelbourn]

LGTM

This reminds me that I want to make a StackBlitz of this demo running and link to it from here

[Splaktar]

@jelbourn here's a StackBlitz of this demo. Did you just want to link it from this PR or from the README of the YouTube Player component?

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