First pass at /sea-levels/ endpoint

Much of this has been taken from the Prediction app. It uses very similar
view and serializer pattern.

- Add database model CombinedPredictionObservation
- Uses a custom SQL view written in the initial migration. The Django
  model is "unmanaged" meaning it doesn't try to modify the database.

This needs an entire test suite before acceptance...

[#75795976]