Improve behaviour for attempted copy of unsaved paragraph

The Copy to clipboard button is now disabled for unsaved paragraphs in the paragraphs_clipboard functionality and hidden in layout_paragraphs_builder.
Enhanced error handling now provides a clear message when attempting to copy an unsaved paragraph: “You cannot copy an item that has not yet been saved.”

Hi @hhvardan. I've tested 3485091-improve-behaviour-for:
* Confirmed the copy to clipboard link is disabled for a new unsaved paragraph using the default widget.
* Confirmed the copy to clipboard icon/button is not shown new unsaved paragraph using the Layout Paragraphs widget.

So, the issue here is fixed.

Additional: I realised while testing, there's a related issue: if you change an existing paragraph without saving, then of course copy to clipboard will copy the original version, not the edited version. I think we have to accept that as a limitation, at least for the initial release. Later, if we can make it to copy the edited version of a paragraph, and maybe even a new one, that would be great.

RTBC see #5.