Clean up API codebase by using idiomatic DRF and removing boilerplate

[dhruvkb]

Fixes

Fixes #181 by @dhruvkb
Partially addresses WordPress/openverse#747 by @dhruvkb

Description

This PR consolidates API endpoints into an inheritance based model using DRF viewsets. This vastly reduces the amount of boilerplate code.

This PR

• replaces APIViews with code repetition into highly abstracted ViewSets
• uses Router to automatically setup URL mappings for APIView endpoints
• uses DRF's blessed approach for pagination
• moves documentation logic outside views to make it easier to maintain
• enforces nomenclature conventions in serializer classes (such as 'Request' for input serializers)
• brings parity between image and audio views (adding report view for audio)
• cleans up exception handling logic for consistent error messages
• performs other housekeeping work such as
  • removing unused helper functions, serializers and utils
  • adding documentation wherever deemed inadequate
  • reducing hardcoding and boilerplate with programmatically generated values
• updates tests so that we have ✅

Checklist

• My pull request has a descriptive title (not a vague title like Update index.md).
• My pull request targets the default branch of the repository (main or master).
• My commit messages follow best practices.
• My code follows the established code style of the repository.
• I added or updated tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no visible errors.

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[krysal]

This is HUGE. The separation of documentation from the app code is really appreciated, and though seems like the extra layers of DRF make the curve steeper for new contributors, I believe the benefits should pay off in the long run, given your exposed points.

I also want to highlight this quote of yours for future PRs:

Preferably make individual atomic PRs addressing one issue at a time, so that they can be reviewed and merged faster.

That said, thanks for all this good work!

[dhruvkb]

Really sorry about the size @krysal, and thanks for the review. I tried making small PRs but all the changes were so interdependent on each other. I'll definitely try harder in the next one.

[zackkrida]

@obulat and/or @sarayourfriend we could use 👀 on this PR; thank you!

[sarayourfriend]

I've tried to review this PR a couple times but I really can't comment on its changes, they're just too many for me to keep track of especially with how little knowledge I have of the pre-existing code. Hopefully @obulat is in a better position to help than I am, sorry!

[obulat]

Great cleanup, and tests pass :) I've learned a lot by reviewing, thank you!

[obulat]

Should these docs go to the make handbook, just like the Catalog and Frontend docs?

[zackkrida]

I don't think so, because they kind of have to live here: https://api.openverse.engineering/v1/ and I don't see a strong reason to duplicate the information. We should however add a link and reference to the API docs in the handbook.