User object privacy (encryption)

As discussed in various occasions, user objects at the platform, despite not accessible anymore by admin/staff users via the web GUI, are still accessible on disk by administrators.

We must setup a scheme to get rid of this "bug" as the trust on the platform itself depends on how well we can keep our users' information.

Here is a suggestion to get the ball rolling:

1. Stored objects have declaration, code and documentation (files) encrypted using the user's password
2. At execution, the complete experiment tree is passed to the scheduler, unencrypted, which relies solely on it (and not any longer on information on disk) to execute the experiment.

Possible issues:

1. How to deal with shared objects (public objects should be left unencrypted, I guess, but privately shared ones are not that clear)? Maybe we'll need to duplicate objects at sharing time.
2. How to effectively implement this?

Side effects:

1. The scheduler would have no longer to verify the hash of submitted experiments.