Create an `openedx_catalog` app with a representation for CourseRuns

[ormsbee]

The Learning Pathways project would benefit from having a representation of CourseRuns in this repo, since the first Pathways will be configured using CourseRuns.

As a migration step, we would push data into this new model from openedx-platform's CourseOverview. At some later point, we can reverse the dependency relationship.

Some high level thoughts/ramblings:

1. We probably want a very small table because it's going to be joined against a lot in the future. Common metadata should go in 1:1 models that we optionally join on. We want to avoid CourseOverview's "just one more field" syndrome, and try to divide up supplementary data into logical pieces like CourseSchedule.
2. For phase one, we only need the minimal set of course data needed for Learning Pathways to query.
3. We could make the initial model more general, e.g. LearningContext, even if we only push the course keys to it for now (and then subclass or 1:1 for CourseRun). I'm not sure if this is really the way we want to go though, because LearningContext feels like it's only a catalog concept by coincidence because of how the system evolved.
4. We should use actual opaque key Django fields.
5. I'm guessing we'll want to use RBAC (opeendx-authz)?