This introduces a new reviewboard.deprecation module, which defines a

base class for deprecation warnings

(BaseRemovedInReviewBoardVersionWarning), a RB4-specific subclass

(RemovedInReviewBoard40Warning), and a stable alias representing the

upcoming version (RemovedInNextReviewBoardVersionWarning).
All current uses of DeprecationWarning have been updated to use these

instead, and all old deprecation warnings have been given new life

through the remainder of 3.0. In 4.0, the deprecated functionality will

be removed.

Unit tests pass.