Integrate with the key module

Looks ok

This needs to be made a lot more robust.. Imagine you have a running site using this module just fine, and then decide to enable the key module.. Suddenly your keys no longer work, as the code in #2 suddenly expects that the configuration values refer to keys with the same name managed by the key module.

The way this should work is to refactor the client id and secret configuration to add a "provider" attribute that indicates the use of the key module. Naturally, the module should then be blocked if the key module is disabled.