fix changing update key in MbNameTag

Close #1571 (closed)

Close #1584 (closed)

(Blocks #1607 (closed)) → sending a file creates so many DOM updates that it crashes my browser

I already spend too much time debugging the source of the changing user object… This is just a simple fix that checks if it is necessary to update the user object, i.e. updating the key and destroying the DOM node.