Decouple "UI configurable settings" from the Django settings

[jacobtylerwalls]

There are some disadvantages to merging the user-configurable settings (SystemSettings) into the Django settings (django.conf.settings).

• It's led to user confusion or difficult issues to debug, see Unexpected SystemSettings interaction causing failure to run via wsgi.py #11660, SEARCH_EXPORT_LIMIT in settings.py ignored by whatever in Arches frontend system settings #11697, incorrect docstring on SystemSettings class
• The code to merge settings into the django settings machinery is:
  • difficult to remove/refactor, introduces circular import problems
  • goes against Django doc guidance to avoid altering settings at runtime
  • goes against Django doc guidance to avoid module-level queries
  • difficult to mock when testing, requires custom decorator to wrap the django decorators for overriding settings
• Introduces the risk of importing the wrong settings object (vanilla or custom?)
• Raises questions about arches application architecture

However, having a friendly user interface for updating a subset of settings is useful!

Suggest in a future version having a friendly user interface for these settings, whether an Arches resource model or not, that is not then merged back into the django settings module.

cc/ @chrabyrd