Add popup message to address common YouTube error reports

[lenacohen]

We consistently receive the most error reports on YouTube (#1555). Since the error reports are rarely caused by Privacy Badger issues, I added a message to the popup addressing common error report complaints. This message links to a new FAQ about YouTube: https://github.com/EFForg/privacybadger-website/pull/134

To test, pull branch and run Privacy Badger locally.

• Visit YouTube and confirm you see the following popup.

• The link won't work until the corresponding website PR is deployed.
• Confirm that you don't see the YouTube message on other domains.

Design and wording feedback welcome! Let me know if there are other popup messages that could appear at the same time so I can test their interactions (the only one I could trigger was the instructions nudge, and it interacted correctly)

[ghostwords]

Merged as d200ff8c7d870b96748652feb1a02e003ccffbdb.