gddo-server: redirect based on config

shouldRedirectRequest now uses data from the DynamicConfig to decide
whether a request should be redirected.

Change-Id: I69a3d4df275248003b170c0e48ca6959cad93603
Reviewed-on: https://go-review.googlesource.com/c/gddo/+/285933
Trust: Julie Qiu <julie@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>