feat: add command to import postcode areas and code to display on choropleth

[joaquimds]

Required for visualising postcode-level data.

To run the command, python manage.py import_postcode_areas, you must first download the geojsonl files from MinIO: https://minio-console-2-c73y.onrender.com/browser/postcodes

I have added postcodes to the Census Output Area boundary type, as I wasn't sure where to group it. We have to group it with something, as displaying all the postcodes at once at low zoom levels will crash the browser.

Any recently imported data set should be visible at the postcode level, as each GenericData is given a point, which should be located within a postcode. However, in my experience a lot of dragging-around is necessary to actually find the postcode, and the areas jump around a little when switching from output area to postcode.

[sentry-io]

🔍 Existing Issues For Review

Your pull request is modifying functions with the following pre-existing issues:

📄 File: hub/graphql/types/model_types.py

Function	Unhandled Issue
choropleth_data_for_source	GraphQLError: Field not found in data source /gra... Event Count: 69
choropleth_data_for_source	GraphQLError: Formula is invalid /graphql Event Count: 57
choropleth_data_for_source	GraphQLError: Cannot set a DataFrame without columns to the column second ... Event Count: 23
choropleth_data_for_source	GraphQLError: Incorrect configuration for choropleth /gra... Event Count: 16
choropleth_data_for_source	GraphQLError: ExternalDataSource matching query does not exist. ... Event Count: 2

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