Provide user info endpoint

As we are going to store more and more data about the users in our local database (active flag since the option to deactivate a user, superuser as a upcoming flag in !265 (closed)) we need a mechanism for the user to load her/his own data, to handle that in the frontend.

The idea is to provide an endpoint as /current-user that will give back all the user information.

It should also be protected, so that only the users very own information are provided. For the moment we don't want to allow to query information about other users here.