gddo-server: handle /_ah/ requests on GAE.
gddo-server: handle /_ah/ requests on GAE.

Request to /_ah/health will get 200 response. Requests to other /_ah/
will get 404.

Change-Id: I06b809b6ee3144d4a3d0983e132b954765c95c1b
Reviewed-on: https://go-review.googlesource.com/21997
Reviewed-by: Stephen Weinberg <9c6f45be7f163ae13ead4fcb29aeb9763375e17f@google.com>

golang/gddo,golang/gddo,sourcegraph/gddo,golang/gddo,segmentio/gddo,segmentio/gddo,sourcegraph/gddo,segmentio/gddo,segmentio/gddo,sourcegraph/gddo,sourcegraph/gddo,golang/gddo