gddo-server: redirect based on rollout

Logic is added to redirect paths based on a rollout percentage.

Change-Id: Ic79f1064ca6af1301d2e12d398dd8fd0e7ece73b
Reviewed-on: https://go-review.googlesource.com/c/gddo/+/285876
Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>