This enhances the conditions support in order to allow choices,

operators, and value fields to allow for more dynamic logic and state.
Choices can now take user-provided keyword arguments in the constructor,

which are saved and made available for use in matching or other

functions. This is handy for including such things as a request or

Review Board LocalSite along with the condition, for use in queries or

other function calls.
ConditionsField and ConditionsWidget now accept these keyword arguments

for choices. They'll apply these to each choice being constructed. Forms

using this widget can set this in the form's constructor.
A choice or operator's value fields can now be dynamically created. The

BaseConditionChoice.default_value_field and

BaseConditionChoice.value_field attributes can now be functions, which

could operate off of the extra state on the choice. This is useful for

creating querysets or initial values based on form-provided data.
Choices can also now make use of existing operators but with

overridden data (such as the operator name displayed to the user). This

is done with a new BaseConditionOperator.with_overrides() method,

which dynamically creates a new subclass with the overridden attributes.

It's a convenience over the caller having to create a bunch of tiny

subclasses just to replace the name.
Upcoming changes will build upon this to offer improved choices and

operators for working with models and querysets.

Unit tests pass.
Tested manually with soem new choices/operators that make use of these

new dynamic capabilities, and verified that they performed as expected.