Simplify edx-platform settings

[kdmccormick]

Intended final settings structure

See https://github.com/openedx/edx-platform/blob/master/docs/decisions/0022-settings-simplification.rst#target-settings-structure-for-edx-platform

Open questions

• When exactly do we "rebase" Tutor's settings onto (lms,cms)/envs/(common,development)? Most likely, we should do it all at once when those four files are ready, as Tutor's settings are all coupled together into a Jinja hierarchy.
• Should the settings object should have same type signature between LMS and CMS?

Some general Testing Notes (Update as we improve this)

• Use the Settings Diffing tooling to check this
• See the testing instructions in previous PRs here:
  • refactor: Clean up lms/envs/production.py cruft #36115
  • refactor: Clean up CMS production settings cruft  #36306

[kdmccormick]

@robrap , @dianakhuang -- I have split this off from the greater Let's Talk About OEP-45 issue. We'll be tracking the ongoing edx-platform settings simplification here.

[kdmccormick]

Ready for review: #36224