Link URI re: issue 486

The proposed changes address the silent failing of adding a link to an unrecognized URI. They allow the user to correct any potential mistakes made to the initial address through a prompt dialogue. 
One caveat here is that you could add a `file:` link and you could do it in groups (!). Good? Bad? (I don't see anything bad about it, except we wouldn't want to encourage adding links to files using this method)