Always display YouTube videos in 16:9 aspect ratio

[AprilSylph]

https://github.com/AprilSylph/Outbox-for-Tumblr/blob/a2e9e4eba365831481d2c6151552199d6f858437/src/outbox.css#L441-L443

This CSS assumes the video width is 540px, which is usually correct. However, post width can shrink according to the viewport. The universally correct value would be:

height: calc(min(100vw, 540px) * (9 / 16));