Fix not found error when editing keyvals

Keyvals are a key and a value. If someone mistypes the key, editing the key with the edit form gives the "not found" error because it tries to edit a non-existent keyval with the new key. Instead, we should disallow changing the key of the keyval from the edit menu. You might want to add another label/unchangable text component to AdminForm to do this. (Unless you want a fancier fix where the old keyval is deleted and a new one created in it's place with the new key.)