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. 
Instead of calling `Zotero.Attachments.linkFromURL` here we should be returning the result to the caller. This makes the dialog more versatile (e.g. we can use it to edit the link as well) and makes sure that all database operations are done once the dialog is closed.