User interface for world location featuring

Add the user interface to manage features independently for each locale of a
world location.

I've introduced a new tabbed view of world locations and improved the
world location index page admin.

The controllers and views for feature lists and features stand alone
from world locations so they could easily be used for managing
featurings on other object types in the future.

The frontend code introduces a feature list presenter to present
features correctly.
Acceptance tests for independent featuring

Test explicitly that we can feature independently on each locale, and
that the correct translation is displayed.

I added an extra css 'feature' class to featured items to facilitate
testing.