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. 
> (@dstillman this can only be used on the main thread, so I'm not sure if you would have an issue with that)

No, that's fine — most things run on the main thread. The coming async/off-thread switchover is just for DB and (to a lesser extent) file access.

And no need to keep a reference to nsIIOService with the ```this.ios```. (But spaces after ``if``, please!)