Create a "zero knowlege" storage option for Passcrow

[BjarniRunar]

If this project takes off, the value of stored secrets may grow significantly, making servers an increasingly tempting target for attackers.

This won't happen right away. But if things take off, we will need to take steps to reduce even further the value of the data stored in a Passcrow server.

One idea is "collaboratively encrypted" storage, that is to say, storage where the data is encrypted in such a way that no single entity alone possesses the keys required to decrypt it. This means all decryption operations would be cooperative, requiring (automated) cross-organizational collaboration, and any attacker who wishes to make use of stolen data will be forced to steal data from multiple targets to have any hope of using the data.

This isn't particularly hard to do, using the same secret-sharing algorithms as Passcrow itself uses. But it's complexity we don't need yet, so the idea is simply recorded here for now.