access: mailcookie port using SQLAlchemy

There is a port of mailcookie to SQLAlchemy. We should maybe open a discussion if we are going to improve it or deprecated in favor of any standard solution based on OAuth, [Flask-Security](https://pythonhosted.org/Flask-Security/api.html#flask_security.utils.get_hmac), or other.
Why having this as a public property, rather than embed it in the data property?