Move source_file to separate class.
Add import functionality for demographic data

This adds a number of classes to track demographic data:
- DemographicDataSource: A shapefile providing demographic data
- DemographicDataSourceProblem: A Problem with a DemographicDataSource
- DemographicDataFieldName: The fields available for import from a
    DemographicDataSource
- DemographicDataFeature: Stores the values of the fields chosen by
    the user along with their associated geometries.
- OTIDemographicConfig: The configuration of the app regarding which
    fields from the shapefile are being used for indicator calculations.

It also adds two Celery tasks for dealing with shapefiles containing
demographic data:
- get_shapefile_fields: Generates DemographicDataFieldName objects based
    on the field names found in the shapefile, but does not import any
    data.
- load_shapefile_data: Loads the data specified by the user from the
    shapefile into the database; creates DemographicDataFeature objects.
    Removes any existing data before running, so only one set of data can
    exist in the database at any given time.
Remove currency from poverty_line.
Flesh out Celery task comments.
Merge branch 'feature/demographic-data-backend' into develop