Circular import in models.system_settings.py

[jacobtylerwalls]

Importing arches.app.models is a bit fragile because arches.app.models.system_settings also imports from arches.app.models import models, which it only needs in a couple of instance methods.

Discovered in #11016, where we worked around this by just avoiding system_settings.py altogether:

Yep, if you don't hide the circular import like this, you get this when transmitting settings.py to nodejs:

  File "/Users/jwalls/prj/arches/arches/app/utils/thumbnail_factory.py", line 48, in <module>
    ThumbnailGeneratorInstance = ThumbnailFactory().create()
                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jwalls/prj/arches/arches/app/utils/thumbnail_factory.py", line 29, in create
    if settings.THUMBNAIL_GENERATOR:
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jwalls/prj/arches/arches/app/models/system_settings.py", line 69, in __getattr__
    self.update_from_db()
  File "/Users/jwalls/prj/arches/arches/app/models/system_settings.py", line 90, in update_from_db
    from arches.app.datatypes.datatypes import DataTypeFactory
  File "/Users/jwalls/prj/arches/arches/app/datatypes/datatypes.py", line 29, in <module>
    from arches.app.utils.permission_backend import user_is_resource_reviewer
  File "/Users/jwalls/prj/arches/arches/app/utils/permission_backend.py", line 19, in <module>
    from arches.app.models.models import ResourceInstance, MapLayer
ImportError: cannot import name 'ResourceInstance' from partially initialized module 'arches.app.models.models' (most likely due to a circular import) (/Users/jwalls/prj/arches/arches/app/models/models.py)

Originally posted by @jacobtylerwalls in #11016 (comment)