Small cleanups to List models

[cdrini]

Refactor; remove some dead/broken code from List models. Includes a small breaking change to public APIs.

Technical

• Remove some dead functions
• Use Python 3.8's cached_property instead of home grown
  • Confirmed this behaves in a similar fashion; see https://docs.python.org/3.9/library/functools.html#functools.cached_property
• Fixed + Re-enabled tests for this file
• Removed dead list/seed properties: work_count, edition_count, ebook_count
  • These values were always 0. Note this is a breaking change to a public API, but considering it was not working and that fixing any broken code is trivial (switch to seed_count), I think it's ok.

Testing

• Locally tested:
  • Lists with works render
    • Lists render
    • .json works
    • seeds.json works
    • All the export options work
  • Lists with subjects/authors:
    • Lists render
    • .json works
    • seeds.json works
    • None of the export options work -- this is the case in prod
• Test on real data on testing
  • Test something with redirects

Screenshot

Stakeholders

[mekarpeles]

changes lgtm, some concerns about whether there are other pages using any of the functionality that we've changed (apis, list page, etc). This is @jimchamp's PR to test but we wanted to look through it as we are also working on ListQuery Carousel changes and wanted to avoid overlap