StringTagValueFragment: Update model for constraints in keyup

[hallahan]

Right now, we test constraints any time a radio button or check box is selected to hide / show other tags. We only test constraints for the StringTagValueFragment when the user tries to save. This means that if there is a constraint that depends on a StringTagValueFragment, it won't really be seen until the user tries to save.

Right now, I'm not seeing any constraints where this actually matters, but it could become a bug. I'm going to keep this low priority until someone cares.