[Server] Initial Prefix Generation/Storage/JWT Encoding

[suyashkumar]

Add a unique random N byte prefix secret to the User model. This should be generated on sign up. (These will have the capacity to be regenerated as well if necessary in a future change). This prefix will be encoded into the User's JWT as well.

This prefix will be prefixed to the name of devices the user controls to namespace it and restrict access to the device (must know the secret to access)