Blocks created by `shuffle` and `decrypt` are signed with a useless signature

[ineiti]

The shuffle and decrypt protocols create a signature of the public key of the
node who did the shuffle/decrypt and put this signature in the block.
However, this signature is pretty useless, as it can easily be copied and re-used!

Fix: sign the actual data that is sent to the chain, if possible add the hash of the previous block to the data which is signed