Add support to have a fixed keypair per user not depending on the password
Current code is deriving the private key from the user password which means that it's one keypair per password. That's an issue when a user changes password because it would mean that you need to re-encrypt all the things with new password using the old one.
Instead, we should find a way to lock the private key from the keypair in some crypto box that is decrypted with the password thus simply storing that blob in the database making it much more easier to change the password as you only have to change that crypto box password in the database.