Overview: Allow -foo to show up on post tag history
Effected area:Post tag history
Current behavior: Only tags that were on the image after the tag update are shown.
Desired behavior: Any point in which the tags list shows the term foo, it will be shown, whether it's a new, pre-existing, or removed tag.
Usefulness: See when tags were removed, too.

After all, we can't just search for +foo and see where the pluses are black, we have to navigate through what is currently added OR pre-existing to find the black pluses.