gddo-server: do not redirect /fonts/* requests

Change-Id: Ib1b089640af5e70defd7625515ee11d961c132da
Reviewed-on: https://go-review.googlesource.com/c/gddo/+/287212
Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>