gddo-server: redirect stdlib

Logic is added for selectively redirecting stdlib pages.

Change-Id: I9ba6cecd0108f2d232e0bbf2de00eb43b18cab89
Reviewed-on: https://go-review.googlesource.com/c/gddo/+/285879
Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>