Run automatically UI tests when opening a PR changing rustdoc/CSS/JS content

[GuillaumeGomez]

I think the change will need to take place in https://github.com/rust-lang/highfive to make things smoother.

cc @QuietMisdreavus

[JohnTitor]

@GuillaumeGomez So, IIUC we expect it's done by this config, right?
https://github.com/rust-lang/highfive/blob/4966ceffefe8eba2756c84e7dc17d5cca9fa431f/highfive/configs/rust-lang/rust.json#L53

But I'm unsure if that config runs ui tests nowadays. And it seems our highfive fails to add the label when opened PRs that touch rustdoc related things, I doubt this is the cause.
Also, this is a nit point but the mention should be @rust-highfive, not @highfive imo.

[GuillaumeGomez]

It's done.