Courses without Circuits

[rosscado]

In the Lucan GP use case (issue #57) the event details page states the distance of the races but not their route/course/circuits.
We must be able to capture the available race distance information when circuits are unavailable. Even if it turns out later to have a discrepancy with the circuit (e.g. if distance_km % circuit.distance_km != 0) that's information we would want to represent too.
Requirement:
Add a distance_km field to the Course model.

Originally posted by @rosscado in #57 (comment)