Update YouTube videos and links to use HTTPS

[samdutton]

Mixed content rules mean YouTube videos don't work with HTTP URLs on HTTPS pages.

For example:

• https://www.html5rocks.com/en/tutorials/speed/animations/#an-example (scroll down)
• https://www.html5rocks.com/en/tutorials/webrtc/basics

I've updated iframe src and link href values.

Whitespace has been changed in some HTML, but this shouldn't have any effect — and shouldn't be too much of a pain reviewing this PR...

[PaulKinlan]

Thanks @samdutton