Instead of hiding rustdoc search bar by default, disable it

[GuillaumeGomez]

It'd have the advantage of not having the not so pretty effect we have currently when the search bar "appears".

[Mark-Simulacrum]

When are we hiding it? Without JS?

[GuillaumeGomez]

No, by default it's not displayed. We make it "appear" when we're done loading the index file.