Recreate tag-deletion functionality in ajax

[waldoj]

It used to be possible for admins (i.e., me) to delete tags directly from the bill page. But that functionality doesn't exist with the new, ajax-based tag-deletion functionality. Also missing is the functionality to delete tags from the admin page (it works, but it terminates in a blank page). I've started on it, but there's more to be done.

• get the [x] links working
• catch the response to verify that it worked
• remove the tag from the DOM when the deletion worked