Reuse sidebar on narrow screens script

[szpak]

It would be useful for #221, but anyway it's good to keep scrips in one place, if possible.

Btw, there were subtle differences between scripts. One version had:

if (document.getElementsByClassName('navbar__menu')[0]) {
    document.getElementsByClassName('navbar__menu')[0].classList.remove('is-active');
}
if (document.getElementsByClassName('mobile-search')[0]) {
    document.getElementsByClassName('mobile-search')[0].classList.add('hide');
}

while another just:

document.getElementsByClassName('navbar__menu')[0].classList.remove('is-active');
document.getElementsByClassName('mobile-search')[0].classList.add('hide');

I kept the first version as it seems to be safer and probably just hasn't been backported to other places.

[zzossig]

Thanks!