ConditionsField.choice_kwargs, which is used to provide custom values

to pass to ConditionChoice instances (for request state or anything

else) was getting shared across multiple ConditionsFields for multiple

instances of the same form. The reason is that the choice_kwargs

wasn't being copied when Form creates instance-specific copies of its

declared form fields.
We now perform the copy, but in the widget (which previously shared

ConditionsField's version). ConditionsField no longer keeps its own

copy, but instead provides a property that wraps the one in the widget.

This ensures there's only ever one version of choice_kwargs that has

to be managed per field per form instance, and that it's always unique

to that instance.

Unit tests pass.
Verified this fixed a bug I was hitting in Review Board unit tests.