Add lightweight feature hiding functionality

Introduces the concept of a "feature" that can be hidden. A list of
features to hide can be supplied in OSCAR_HIDDEN_FEATURES.

Feature hiding is currently supported in two places.
Application instances can carry a "hidable_feature_name". If set and
found in OSCAR_HIDDEN_FEATURES, post_process_urls will return an empty
set of url patterns, hence disabling relevant views.
display_tags also has a new template block tag iffeature.
{% iffeature "feature" %}xxx{% endiffeature %} will hide everything
inside the block if the feature is hidden.
Make reviews app hidable

Using the newly introduced feature hiding functionality, the reviews app
can now be hidden.
Adjust test generator to make tests pass