Fix FakeJob (executes functions synchronously)

PR #234 (replace RabbitMQ by Redis) introduced an asynchronous job abstraction. A way to execute async jobs synchronously was foreseen, but broken due to a signature change in `rq.Job`.

This PR fixes FakeJob and furthermore introduces the configuration option `adhocracy.background_processing` which allows to disable background processing entirely.

This would also allow to execute the test suite without running Redis by changing `test.ini` (either unset one of the `adhocracy.redis` options or use `adhocracy.background_processing = false`), but this is for another PR and needs testing first.