Generic user hash

Sometimes we need to generate our own hash value for a user. To implement this:

• Add PEPPER as a system config value
• Add salt as an attribute on the User model (populated when User object is created)
• Add hash as a function on the User model:

def hash(input, hash_function="bcrypt"):
    if hash_function == "bcrypt":
        return bcrypt.hashpw(input, user.salt + system.PEPPER)
    else:
        throw UnknownFunctionError