Fix the default feature checker to work if the request doesn't have a LocalSite

Review Request #850 — Created Sept. 4, 2017 and updated — Latest diff uploaded

Review Board
Some of our URLs won't end up attaching a local_site attribute to the request
(for example, pages in the admin site, or the manual updates required page). In
this case, we still want `{% is_feature_enabled %}` to work without crashing.

Testing done:
- Called the `is_feature_enabled` tag from the base template and was able to view
  pages that crashed before.
- Ran unit tests.

Reviewed at