Allow YouTube embed to use /channel/live links

[revelsix]

As of now, the only way to embed a YouTube stream is using the actual video URL. If possible, being able to use the /live link, (https://youtube.com/user/destiny/live, where destiny can be replaced with the a channel URL) would make embedding YouTube links about as easy as embedding Twitch links.

A couple immediate problems with this:

1. This would only be useful for channels with custom YouTube URLs, as default channel URLs are actually longer than video links.
2. There are a couple edge cases in which a channel has multiple streams running, where the latest stream appears on the /live page
3. (!) There are 3 types of URLs, /channel/ default long URLs, /user/ legacy custom URLs, /c/ new format for custom URLs, typing youtube.com/[person] will prioritize /user/ links, which can cause problems when 2 channels use the same URL (ex: https://youtube.com/chilledcow points to a /user/ channel with 90 subs while https://youtube.com/c/chilledcow/ brings you to a 6M subscriber channel)

[11k]

Is a custom persistent broadcast URL embeddable? I did some research and it seems the best we can do is https://www.youtube.com/embed/live_stream?channel=<channel_id>. This means we'd have to resolve usernames to channel IDs to make this work, which is a nontrivial problem. The Search API can't be used because of the extreme quota cost. I wonder if this user is correct when they say that the forUsername filter only works for legacy username URLs.