separate authn and authz into two different subsystems

## Expected Behavior
Be able to have one Plugin configured for authentication (authn) and another for authorization (authz).

## Current Behavior
Authn and authz are bound into one subsystem.

Only with LDAP authentication and the group-role mapping it is possible to have the lookup for authentication and the lookup for authorization together.

## Possible Solution
Split authentication and authorization in two different sections and combine them to the customer login and authorization model.

That will be more complex but gives the ability work around the different security models that are used with Graylog.

## Context
When using SSO for authentication you are not able to use a proper authorization. Currently the only option would be to build a custom plugin that hold the authn part and the authz part and combine them.