net/http: make FileServer look good on mobile

net/http: make FileServer look good on mobile

Currently when viewing directories on a phone, the text is small and
often hard to tap correctly.  This commit adds the viewport property to
the page to make it look correct on phones.  This commit also makes the
page behave in Standards Mode instead of Quirks Mode which does not
effect the behavior of this page but makes me feel good inside ☺️

Change-Id: I4babcf79085e85fba57453b7a235e4750a269a42
Reviewed-on: https://go-review.googlesource.com/c/go/+/552595
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Commit-Queue: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Max Coplan <mchcopl@gmail.com>