Add database model CombinedPredictionObservation

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

[#75795976]
First pass at /sea-levels/ endpoint

Much of this has been taken from the Prediction app. It uses very
similar view and serializer pattern.
Merge pull request #68 from sealevelresearch/add-combined-endpoint

First pass at /sea-levels/ endpoint