Registry instances are commonly constructed in the __init__.py of a

module, but this can interfere with the Django initialization process if

the registry's module or population logic requires direct or indirect

access to modules containing models or some other Django apps.
The solution to this is to lazily construct the registries on access.

This can be done through SimpleLazyObject, which will construct an

instance on first access and then forward on all operations to it. To

simplify usage of this, this change introduces a

lazy_import_registry() method, which will set up a function to import

and construct of the registry and return a SimpleLazyObject with that

function.
This should be usable in any place where a registry needs to be

constructed. It can also help speed up startup times when working with

more expensive registries.

Made use of this in Review Board. All unit tests pass there on all

supported versions of Django.