[rbdemo] Update for Review Board 4.0 and Python 3 compatibility.
Review Request #11068 — Created July 5, 2020 and submitted
This change gets rbdemo in shape for Review Board 4.0. These consist of
some changes around the structure for management commands, a change for
an app label, the removal of an argument to the
command, and modern octal format support.
Tested outputting new demo fixtures from an upgraded copy of the demo
database and diffing results from what we use in production.
Tested resetting my local demo database based an demo fixtures and
verifying the results.