Implement YouTube Click to Load

[kzar]

Add privacy protection from embedded YouTube videos and
playlists. Block third-party YouTube requests and display a
placeholder for the corresponding element that allows users to
unblock the content again. Where possible, also upgrade embedded
videos to use youtube-nocookie.com.

Reviewer: @jonathanKingston

CC: @ladamski, @kdzwinel, @Charlie-belmer, @sammacbeth

Steps to test this PR:

1. Add the following entry to your social_ctp_configuration.json file: https://gist.github.com/kzar/89bc3b8971daef09334da5a0799589c0
2. Navigate to the YouTube test page and follow the instructions: https://privacy-test-pages.glitch.me/privacy-protections/youtube-click-to-load/

Automated tests:

• Unit tests
• Integration tests

Reviewer Checklist:

• Ensure the PR solves the problem
• Review every line of code
• Ensure the PR does no harm by testing the changes thoroughly
• Get help if you're uncomfortable with any of the above!
• Determine if there are any quick wins that improve the implementation

PR Author Checklist:

• Get advice or leverage existing code
• Agree on technical approach with reviewer (if the changes are nuanced)
• Ensure that there is a testing strategy (and documented non-automated tests)
• Ensure there is a documented monitoring strategy (if necessary)
• Consider systems implications

[jonathanKingston]

Looking good, I left some general comments but haven't tested it out yet. :)

[kzar]

Thanks for the feedback!

[sammacbeth]

Looks great! I just have some minor formatting comments, otherwise I don't see anything major. If you could fix the console errors in Firefox I think we'll be good to merge.

[sammacbeth]

Thanks for the fixes. LGTM