Only youtu.be links worked: watch links sent the video id "watch", shorts
links "shorts", and a link with no path threw while building the video
page. The link is now read for its video id and start time in every common
form, and the embed URL is built with the id and time as encoded query
parameters.

The video view also stays on the embed page: links that would replace it,
like the player's own YouTube links, open in the browser.