Migrate everything to the new statistics API / choropleth UX smoothing

[janbaykara]

Description

From a user perspective, this PR makes configuring choropleths more self-evident, and less janky/buggy/gotcha'd.

The three standard modes (count, field and formula) are complemented by a fourth: "advanced", which opens up a popup for you to edit the more complicated and fanciful options we've added recently.

Backend

In the backend, there's been a bit of refactoring. The map and explorer now always uses the same statistical maths and querying logic:

• We've removed the old choroplethDataForSource and genericDataSummaryFromSourceAboutArea resolvers and moved everything to the new statistics method (resolved by statistics and statisticsForChoropleth).
• There's a new filter_generic_data_using_gss_code function.
• Abstracted calculated values (like first, second) into add_computed_columns.

How Can It Be Tested?

• Firstly, existing map reports in your localhost should upgrade without a hitch via the migration code: choropleths should generally continue to work just as they were previously configured.
• Load up a list of members: count mode should work without the advanced editor.
• Load up a list of area stats: fields should be self-evidently configurable now without the advanced editor.
• Similarly, formulas.
• Open up the advanced editor and do something fancy. For example, load the GE2024 ward results, select Nominal data type, then shade by \Winner (2024)`(including the backticks), and enableAre these electoral parties?`: you should see a party winner map!
• Explorer displays should work as confusingly as they did before — reorganising them so they reliably show what we want is a bit out of the scope of this PR

[linear]

MAP-1005 Migrate over to new statistics API

[joaquimds]

lgtm, thanks Jan 👍

[sentry-io]

Suspect Issues

This pull request was deployed and Sentry observed the following issues:

• ‼️ GraphQLError: "['count'] not in index" /graphql View Issue

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