Use an environment variable to determine whether to use GCS.
Use an environment variable to determine whether to use GCS.

By default (as per now), we'll use Google Cloud Storage for listing and
fetching the downloads. However, if DISABLE_GCS is set (to any value),
we'll avoid initialising GCS and instead rely on the fake repositories.

This allows one to run:

$ DISABLE_GCS=1 dotnet run

without any additional code changes, and get a working local site.

Also avoid registration of the GoogleCredential singleton if GCS is
disabled, since it's only used by the GCS-backed repositories, and has a
non-trivial setup.

malcolmr/nodatime,nodatime/nodatime,malcolmr/nodatime,jskeet/nodatime,BenJenkinson/nodatime,malcolmr/nodatime,jskeet/nodatime,malcolmr/nodatime,nodatime/nodatime,BenJenkinson/nodatime