devilry_detektor: View that works with a basic celery task.

See #595.
devilry_gradingsystem: Removed wrongly placed templates for devilry_detektor.
devilry_detektor: Re-added templates (in the correct app).

See #595.
devilry_detektor: Added to default urls.

See #595.
devilry_developer: Set celery settings for tests.