Don't attempt to cascade a delete from a view

The CombinedPredictionObservation model is actually not managed by
Django as it's a SQL view onto the database. When a Location is deleted
it shouldn't cascade and attempt to delete the
CombinedPredictionObservation.
Add import_surge_predictions management command

- New app called surge_model_converter
- Mapping between ie `LVPL` and our internal Locations
- Database migration to make this mapping table and populate with
  Liverpool
- import_surge_predictions can keep track of which files is has already
  loaded

https://www.pivotaltracker.com/story/show/75795728 [Delivers #75795728]