content/static/css: hide pathInput element on larger displays
content/static/css: hide pathInput element on larger displays

"DetailsHeader-pathInput" input, used for copying package path, was
showing up on larger displays than 1080p.
This commit fixes it by using display's width (vw) instead of rem

Fixes golang/go#38162

Change-Id: Ide97e0308eb0cd56131507b933a7f3ed889ffc16
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/238477
Reviewed-by: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite