Allow to split mediacenter and staticpages services

This allows to use separate mediacenter and staticpages API endpoints by
defining the more special configuration options:

* adhocracy_service.mediacenter.rest_api_address
* adhocracy_service.mediacenter.rest_api_token
* adhocracy_service.staticpages.rest_api_address
* adhocracy_service.staticpages.rest_api_token