Series and external work: Add descriptive classes to series' & external works' stats in bookmarks

In bookmarks, works, external works, and series all have a dl list for the bookmarked objects stats. However, while for a work bookmark the dd and dt in that list have descriptive classes like "bookmarks" or "words" that identify what information they are displaying, the series and external works bookmarks are missing these classes (the dd and dt are bare).

This complicates styling with site skins because you can't target all instances of a stat when some of them are missing the relevant class.

Solution: add the relevant classes (e.g. "words", "bookmarks", or "works") to the HTML for these stats in the bookmark blurbs.