WIP: Unify names of generic relations to media classes

[sarayourfriend]

Fixes

Fixes #4553 by @sarayourfriend

Description

This is still WIP because I need to test a bit more thoroughly. So far things appear to work with manual testing, but I haven't run unit tests, and I suspect there are edge cases in the model admins that I have missed.

Heads up through, @dhruvkb, if you want to take a look at the approach I've taken. It's a bit abstract, and I could definitely see wanting to use a more concrete, straightforward, declarative approach that just manually creates the back references onto the media classes.

The approach overrides the metaclass for Django models with one specifically for models related to a media type. The abstract models reference that metaclass, along with a related name, and then subclasses are processed to get assigned to the media class as {related_name}_class, and the related name from the abstract class is passed down to the Meta of the final models (which Django doesn't do on its own, only a small set of fields are inherited from Meta by default).

I initially tried this with __init_subclasses__ but you can kind of see what that results in with the admin models and forms... but a bit more unwieldy because of the related name. It ended up being a lot of specific, fiddly implementation details on each of the related abstract classes, which the metaclass was able to tidy up into a single implementation pretty nicely. Of course, it is a bit magical, so if we want to just, say, hand-maintain a nested dictionary of these relations, kind of like the media_type_config fixture in the tests (which could be replaced or reduced by some of this work), and prefer that for its straightforwardness (if being slightly more clunky and requiring more manual configuration of relation names) then I can probably be convinced.

My goal was to prevent any SQL migrations, and indeed, sqlmigrate shows the current migration as a complete noop.

Anyway, let me know what you think, Dhruv, if you get a chance to look at this while it's still a draft. Very curious to see what you'll think of this introspective/meta-programming based approach, and whether you can think of an easier way to do it or think a more concrete and less meta approach would be better.

Testing Instructions

TBD.

Checklist

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• [ ] I ran the DAG documentation generator (./ov just catalog/generate-docs for catalog PRs) or the media properties generator (./ov just catalog/generate-docs media-props for the catalog or ./ov just api/generate-docs for the API) where applicable.

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