Allow embedded YouTube content to display on proxied pages

[mattdricker]

This is an extension of hypothesis/viahtml#23

User has requested that when his course page is fetched through Via, the embedded YouTube videos not be blocked by Via policy.

See https://courses.cs.washington.edu/courses/cse373/23wi/lessons/sorting-algorithms/ loaded directly versus loading in Via: https://via.hypothes.is/https://courses.cs.washington.edu/courses/cse373/23wi/lessons/sorting-algorithms/

[robertknight]

YouTube is blocked in Via to avoid the server-side resource usage of proxying video. To enable YouTube embeds to work we'd need to find a way to either avoid proxying the video player and content, while proxying the rest of the page, or significantly reduce the overhead of proxying. Aside from legitimate use of YouTube, I think in the past we have also had to contend with people using the Via proxy as a way to get around YouTube being blocked in certain places. In fact I think that might have been made the main issue.

[robertknight]

Example issue that motivated the block: #626