Add a deprecation warning for features being removed in RB 5.0.
Review Request #10492 — Created April 2, 2019 and submitted
Review Board 4.0 is deprecating some functionality, with plans to remove
in Review Board 5.0. To help flag these deprecations, we're introducing
a new deprecation warning,
RemovedInReviewBoard50Warning, which we can
use to warn when using any of these deprecated features.
Unit tests pass.
Used this in an upcoming change.