This introduces a new set of choices and values for working with

database models, and a new operator for working with "is not one of"

checks.
The choices and values make it easy to set up conditions that check

against the database in some form. The values can be supplied with a

queryset or a callable returning a queryset. The choice can be supplied

with a queryset or the subclass can override get_queryset(). Combined

with the new support for attaching extra state the conditions widget

and choices, this allows for much more specialized queries that take

into account things like access permissions for database objects.
A new operator, IsNotOneOfOperator, was also added for checking if an

item is not in a list of other items. This is the opposite of

OneOfOperator, which has been renamed to IsOneOfOperator for

consistency.

Unit tests pass.
Used the new conditions, operators, and values in some upcoming changes

for Review Board.