Feature/add osm download

This merge request adds necessary models, tasks, and
views to import OpenStreetMap data. The intention
is to kick off the import process once the GTFS
feed is validated and imported.

The angular/front-end code to kick off the job to
import OSM data is going to wait on finishing the
upload polling directive since my changes will be
touching the same code.

I did not write any tests for the celery task because doing
so requires a successful import of GTFS data which we
do not have mocked up at the moment.