Add to_field_name=slug on LocationForm.parent, update prepare_cloned_fields to handle it

[glennmatthews]

Closes: #2470

What's Changed

• Added to_field_name="slug" on the LocationForm parent field so that the slug, rather than the PK, of the parent Location is used when auto-generating a slug for the new/edited Location. (Fix Child location's slug is autogenerated using parent's UUID rather than slug #2470)
• By itself, this would reintroduce Editing a Location doesn't retrieve parent #2311, because prepare_cloned_fields (which is used to generate the query parameters that are used to pre-populate an edit form in various cases) always uses the PK of any foreign-key fields in the generated query parameters, and passing a PK in the query parameters when the form field expects a slug wouldn't work. Therefore, I extended prepare_cloned_fields to look up the associated edit form, and in the case where a FK model field has a form field with to_field_name defined, use that field from the related object instead of its PK.
• Added a bit more exception handling in DynamicModelChoiceMixin.get_bound_field because I tripped over a ValidationError when initially exploring this issue.
• Enhanced get_related_class_for_model to accept a model instance as a valid input.

TODO

• Explanation of Change(s)
• Added change log fragment(s) (for more information see the documentation)
• Attached Screenshots, Payload Example
• Unit, Integration Tests - there are currently no tests that I see for prepare_cloned_fields; I'll see about adding some
• n/a Documentation Updates (when adding/changing features)
• n/a Example Plugin Updates (when adding/changing features)
• Outline Remaining Work, Constraints from Design