[feature request] Button to copy original URL in documents

When using the extension in pdfs, the url is some ininteligible string of things. I open most pdfs with hypothesis but when I want to bookmark them or send the link to another person, I have to click the button in the extension so I open the document without it, then copy the url or bookmark it and then go to the hypothesis viewer again.
This is very inefficient, it would be nice if there were a button to copy the original url so I can send it or bookmark the webpage overwriting the url with the one I just copied.

Start by locating the extension's PDF handling and the UI for document actions, then determine how the original URL can be accessed. Done means a button copies the original document URL so it can be bookmarked or shared without leaving the Hypothesis viewer.

Written by the indexing model from the issue text.