The LocalSiteFeatureChecker allows features to be enabled or disabled

at the LocalSite level. Features not configured on a LocalSite will

fall back to the global configuration (first site configuration, then

Django settings).
Additionally, the status update work has been reworked to provide the

necessary information (a LocalSite or a HttpRequest) to use the new

feature checker.
This feature checker is now the default for Review Board.

Ran unit tests.