Merge pull request #61 from sealevelresearch/factor-out-common-code

Factor out helper code from prediction app
Add database index on Minute.datetime
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.