internal/short: allow relative paths from the shortend URLs

golang.org/s/key URL shortener supported only simple redirects
from key to a link. This change allows the short link to have
extra path elements, and computes the redirects by appending
the extra path elements to the resolved link. For example,
if golang.org/s/foo is configured to be resolved to example.com,
golang.org/s/foo/bar will result in a redirects to
example.com/bar.

Change-Id: I6aa9f4aab2d5a74c76fda446a29aae998fe48ad6
Reviewed-on: https://go-review.googlesource.com/c/website/+/227654
Run-TryBot: Andrew Bonventre <andybons@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Bonventre <andybons@golang.org>