Restore settings.FEATURES for Django templates

This was removed following the recent merge from upstream. It caused
certain pages (namely the Wiki) to be styled incorrectly (using edX's
default styling).

This fix is not perfect, as some pages now inherit edX styles where they
should not (password reset). This is deemed as acceptable, as it is a
much less jarring user experience.

TODO: Fix these noted exceptions
TODO: edX should move all instance-specific styling to their own theme.