Add "Copy URL" button to the Private URL and Anonymous Private URL modal window

[wfenrich]

The window is currently lacking "Copy URL" button that would copy the URL into clipboard, while its presence seems to be natural.
We should add one somewhere here:

PL label: Skopiuj URL
The button should be visible only if there is an active URL – there is no point in showing it if there is nothing to copy to clipboard.
Nice to have: a little message saying "Copied!" (PL: "Skopiowano!") visible by the button after it is successfully used.