[e.cash] Add youtube to CSP

Summary: Adding youtube.com to the next config CSP so embedded youtube videos dont get blocked.

Test Plan: Nothing to test against atm, the video was removed from the blog since it was getting blocked. Can add back if you need to see the blocking for testing

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D18765