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. 
These should be moved to JS (see my other note), where the second one already exists. The first one should be ```To attach a link to a file, please use "Attach Link to File…".``` You can programmatically get the existing string for that function from the toolbar menu.