cmd/golangorg: fix and clean up m.golang.org redirect
cmd/golangorg: fix and clean up m.golang.org redirect

Use http.RedirectHandler (forgot that existed before).
And add to validHosts so that the general golang.org doesn't kick in.

Change-Id: I80e4cd3f2324889bbda0ea42c17f73695c061abe
Reviewed-on: https://go-review.googlesource.com/c/website/+/342889
Trust: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

golang/website,golang/website,golang/website,golang/website,golang/website,golang/website,golang/website