Because of YouTube's overwhelming dominance in the online video space, we should help our users who paste bad URLs when possible. It is easy to fix `http://youtube.com/embed/acb1233` URLs by turning them into `http://youtube.com/watch?v=abc1233` embeds using an embed handler (we already have one to fix Google Video embeds).

We should avoid doing this in almost every other case, but it's YouTube, and it's an easy error for a novice to make.

Fixes #24660.

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