Devlog is a blockchain too: implement session signing

[mihael]

I don't want anyone to be able to change the devlog I wrote. Only the author should be able to.

Goals:

1. Sign every session cryptographically and insert the signature into the devlog file it self.
2. Devise a "proof of time" or "pot" algorithm to check the devlog signatures.

Future goal: Think about being payed for session time in BTC utilizing the devlog as proof of time or record of work.