Document how to create the default GCS bucket in GAE

Just because I've just spent half an hour looking for it, so I
guess users will have the same problem.
Use GAE's file pkg to find the default bucket.

We were previously hardcoding its value to the FQDN of the app, but
file.DefaultBucketName makes a remote call to determine it, so the
algorithm might change at any time without previous notice. This
change future-proofs the code for determining th default bucket.
Make App.Prepare() idempotent

Saves a lot of logic in multiple places to distinguish GAE to
non-GAE environments, since our main_appengine.go implementation
for GAE already calls App.Prepare()