gddo-server: redirect path prefixes

Logic is added to redirect requests based on a provided URL prefix.

Change-Id: I199ec917b6c21c781299cfb9b9a41b4455a7f345
Reviewed-on: https://go-review.googlesource.com/c/gddo/+/285880
Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>