Djblets has a lot of useful deprecation code, from the warning classes

to the argument checkers, which currently all explicitly specify

"Djblets" in any messages.
These are now built to be consumed. A new

BaseRemovedInProductVersionWarning class has been added, which

contains the base warning code, and introduces a product field.

BaseRemovedInDjbletsVersionWarning now subclasses this, filling in

product.
Both deprecated_arg_value() and deprecate_non_keyword_only_args()

can be used by consumers along with the new base class.

Unit tests passed.
Made use of the new deprecation abilities in Review Board.