This change modernizes the rbslack extension to work with Review Board

4.0 and Django 1.11. This involves updating the urlpatterns and the

handler for the review_request_closed signal, and fixing up

documentation throughout.
I've also decided to add a warning to the extension's description that

it is deprecated and will be removed soon, so hopefully all this work is

actually moot.

Loaded the slack integration and saw that things worked as expected.