Implement django-natural-keys as base for BaseModel

[jathanism]

Proposed Changes

Base models should inherit from django-natural-keys base NaturalKeyModel and use NaturalKeyModelManager so that…

• natural_key() is automatically provided using the uniqueness constraints
  • Currently for each model, we must define natural_key() method
• Foo.objects.get_by_natural_key() is also automatically provided, reducing boilerplate
  • Currently for each model, we must subclass RestrictedQuerySet in order to publish a get_by_natural_key() method
  • And then set objects = FooRestrictedQuerySet.as_manager() on the model
• Each model will also receive a natural_key_slug that is immutable derived from the natural key fields for that object. This can be used in URL fragments in the "natural key or PK" pattern we have begun to adopt for filters.

Justification

We have decided to align around natural keys. We need less boilerplate, not more. In the common case, this should work automatically and result in objects having deterministic uniqueness/natural key constraints with minimal effort. For the uncommon/edge case, we will need to adapt and revise the pattern.

[bryanculver]

#2692

[glennmatthews]

Since django-natural-keys provides a minimal/naive natural_key_slug implementation (for one, it doesn't account for the possibility that any of the key components may contain a - character), we probably need to come up with our own design here. Since one of the main uses of natural-key-slugs may be in URL patterns, we need something that's URL-safe - therefore I'm proposing that we consider either percent-encoding or punycode as a starting point.

[bryanculver]

Completed with #3423