Add featured item for the featured topics and policies lists

This involves a lot of duplication between feature_list and featured_topics_and_policies_list and between feature and featured_item.  Thing is, they're not exactly the same: feature_list & feature are only for featuring documents on other editions whereas feature_topics_and_policies_lists are for featuring any type of content on an org (despite the name suggesting it's only topics or policies).
Allow sortable behaviour to take a selector for finding the ordering label

Via a data attribute, the default is just 'label' which results in it picking the for attr from the first label in the sort-item.
Present a sortable interface for selecting featured items

The user gets a choice of topic or policy and choosing will hide the dropdown for the choice they didn't make.  They can also drag the boxes around to reorder.  The form will always present one empty featured item so you can add new ones.

Because of a bug in chosen (https://github.com/harvesthq/chosen/issues/92) we have to only hide the dropdowns once they are all ready or the one hidden by default will end up being rendered incredibly narrow.
Show the featured items on an executive office page

We cope with featured items for a document that don't resolve to a published edition by collecting all the links and tossing away any that are nil.
Steps for featuring topics and policies on an exec office