A site maintainer can bulk delete all terms in a vocabulary that are referenced by N or fewer nodes
requested to merge issue/cleantaxonomy-3311325:3311325-bulk-delete-unused-underused-terms into 2.0.x
This adds a "Prune terms" menu item and a Prune tab that allows selection of a vocabulary and 'depth' to delete terms that are unused, referenced by one node, or two, or three.
The deletion is then done in a batch process (which is based on the form batch process, which is annoying as it makes doing the drush version of this extra work, which i have not done yet).
Did a bit of refactoring of other code to make use of it here— the list page is now generated by a single query which probably possible to make it user-sortable (it is sorted by usage count and then term ID now).
Closes #3311325