Add term class to largo_top_term() output, prevent largo_category_and_tags() from using wrong post in fallback scenario.

[benlk]

Changes:

• For largo_category_and_tags does not accept specification of an arbitrary post ID #1647, prevent largo_top_term() from calling largo_category_and_tags() when the post ID passed to largo_top_term() does not match get_the_ID's post ID, because there is presently no way to pass that ID to largo_category_and_tags().
• For Add identifying class to top term output, to enable term-specific styles #1646, adds taxonomy-slug output as class on largo_top_term() for the top term, and in largo_category_and_tags() to support that output in a fallback case.
• Fixes problem in the homepage featured stories zone where the post ID was not properly specified to largo_top_term(), and changed to largo_maybe_top_term() while I was in there. This means that largo_top_term() will have the specified post ID in that zone, and in combination with the largo_category_and_tags() change above, if there's no top term set on a post, there'll be no empty h5 element there.

[benlk]

To do:

• update changelog for this
• tests pass