Add comment stats that allow selection of per-group comments

[patcon]

This PR is about reproducing all the output of the repness key in the polis /api/v3/math/pca2 endpoint. Once this is done, all the existing pieces can be used to generate consensus key, but that will be another PR.

Needs a lot of work and cleanup. Much of functionality is in debug.py right now.

To Dos

• port all of repness generating functions from Polis
• add unit test of repness against small conversation from polismath
• clean up code to make more legible
• add polismath types for type checking

[patcon]

Very messy first stub for reproducing Polis' per-group representative statements 🎉
https://gist.github.com/patcon/26e47b3b63a4d8e6b1bd9a94bd70d362

Need to extract the debug.py logic into its own performant functions, and maybe start teasing apart utils.py because it's getting large and complex.

We're skipping concern with the clustering here by extracting cluster_labels from Polis API. Since we're not using the harder-to-reproduce clustering data for now, I can confirm our calculated stats match the Polis API math output with high precision.

[patcon]

This is done and with tests. Will restructure gigantic utils.py in another PR :)