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. 
Oh, we should probably prevent ```file:``` links — not because we don't want to files in groups, but because this isn't a good way to add links to files. It's annoying to do and doesn't do the various nice things that actual linked files do (full-text indexing, content type detection).

There's no point allowing it just as a bad workaround to a current limitation. We should just fix the limitation (by implementing group-specific base directories).