User model: Refactor the config handling

[msm-code]

Part of #23

Right now we handle config very roughly. For example let's take:

auth_default_roles = db.get_mquery_config_key("auth_default_roles")
if auth_default_roles is None:
    default_roles = []
else:
    default_roles = [
        role.strip() for role in auth_default_roles.split(",")
    ]

1. Default value is not declared anywhere, just used ad-hoc
2. The code parses the config key at the point of usage. This obscures the intention and may lead to duplication
3. Last but not least, the value is not validated when changed by the user. And typos here may lock users out and even make the system unusable.

I think about something like:

default_roles = db.config.auth_default_roles 

Where the auth_default_roles is a property that does the magic (parsing and validation)

[msm-cert]

in other words:

Change all instances of db.get_mquery_config(...) to something like db.get_config that returns a well-typed Python object.