Fix dialog reappearing after hitting Enter
Fixes https://github.com/vector-im/riot-web/issues/3714
https://github.com/vector-im/riot-web/issues/3714#issuecomment-297460620 :
> It's as if there are two dialogs and as one closes, the other one appears. For some reason matrix-org/matrix-react-sdk#822 is causing this.
> I've realised it's because the `priorActiveElement` is probably the button that opened the dialog. If this is focused and the enter key is released, this triggers a keyPress which fires once the dialog has closed and the button has been focused :grimacing: the BaseDialog only calls stopPropagation _onKeyDown.
The soln. was to submit the dialog as finished `onKeyUp`. This means the `priorActiveElement` is focussed after any key events that should be associated with the dialog.
