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]
Merge pull request #87 from sealevelresearch/import-surge-predictions

Add import_surge_predictions management command