Add key to registerHtml

[brianjhanson]

Ensures the fetch call will only happen once per page load

[AugustMiller]

Are we seeing this happening in the wild? 😮 It's typically safe to use {% js %}, {% css %}, {% html %}, and the underlying craft\web\View "register" methods with the same strings multiple times… in each case, the fragments are held under md5 hashes of their content, which I would expect to be identical!