Fix Error 153 on YouTube videos

[mike-allaway]

YouTube videos are now all showing as 'Error 153' which stops them from being embedded and the user has to click a link to view the video directly on YouTube.

I found adding the attribute: referrerpolicy="strict-origin-when-cross-origin" to the embed code fixes it.

[benzkji]

there is a PR: #217

[benzkji]

Easy fix is to manually override the embed_video/embed_code.html template, as in the PR.