Return a JSON Web Token instead of the hash #22

The first implementation of the server will use the same mechanism as the Python server: it computes a sha1 hash of the uploaded content and send it back after the upload has succeeded.

Instead we should return a JSON Web Token that the uploader could keep to act on the uploaded data later on.

The payload could be { token: XXXX } where token is the name of the path in cloud storage, and also the URL in /public URLs. It's currently a hash of the content but this will likely change.

Also this JWT shouldn't expire earlier than the data -- currently the data does not expire so the JWT should never expire.

Lastly in this issue we should provide a CLI tool to extract the token from the JWT (without validating it). From my light research I think this should be done with a Python script that has all the necessary utils to do it without installing extra packages, especially: base64 decoding, and JSON parsing.