Use Google Cloud Storage rather than Blobstore for images

[FloEdelmann]

When running the server locally using dev_appserver.py, saving fetched images fails (see traceback).

As it seems, Blobstore uses the deprecated Files API. Also, Blobstore is listed in the Superseded Storage Solutions and its support page suggests using Google Cloud Storage instead.

[peternewman]

I think we got an email about this some time ago, I must confess it's not got to the top of my job list. The whole rdm-app probably needs some love.

Patches welcome. :)

[peternewman]

I'm not sure if this is relevant for us: https://cloud.google.com/appengine/docs/standard/python/blobstore/#Python_Using_the_Blobstore_API_with_Google_Cloud_Storage