Make post-review show a deprecation warning.

This adds a deprecation warning to the 0.5.x branch, to warn people that
post-review is going away in the next major release.

Testing done:
Installed from the release-0.5.x branch and ran post-review. Saw the
warning.

Reviewed at https://reviews.reviewboard.org/r/5201/
Merge branch 'release-0.5.x'