We have a lot of stuff that needs to behave differently in a development environment than in production. We initially conditioned all of these on the `DEBUG` setting, but a lot of users tend to turn that on when they encounter problems, and then sometimes forget that they did so. On the Review Board side, we've moved these things behind a new settings key, `PRODUCTION`. This allows people to set `DEBUG = True` and get more thorough logging, while not completely breaking their system. This change brings that same key to Djblets. Testing Done: - Ran unit tests. - Smoke tested extension template loading.