fix outgoing_links JS error on click; add outgoing_links UI tests (bug 1088435)

https://bugzilla.mozilla.org/show_bug.cgi?id=1088435

flue update: https://github.com/mozilla/flue/pull/23

r? @diox @ngokevin 
super pedant but the comment should say "The URL is" or something like that, it hasn't changed yet.