Add transition between download and continue buttons during LoginFile download

[sisou]

Added in #424, users now always have to confirm that they downloaded the LoginFile. Two buttons in that UI change states after a download has probably happened. For users where the browser does not open a popup for storing the file, a smooth transition between the button states (text on the download button fading out, button shrinking - text on the continue button fading in, button growing) would be great.