Adding tags works now. But it's clunky. Every time the tag field ends editing, the view model tries to add all the tags again. Only new ones succeed because the tagger checks if they're already present.
Adds model code to remove a tag by name from a particular item (by deleting the associated useritemtag).
Caches tokens every time the tag field editing is completed, then sends the old and new tokens to the view model. The view model adds user item tags for the ones only in the new array and removes the user item tags for the ones only in the old array.