Only recognize common url schemes in notification messages

Before the fix, the message tray highlighted all urls containing "://", even
invalid ones. This change fixes this by have the message tray highlight only
the urls with http, https, ftp schemes.

Credit goes to: Phuong Vu, Liye Fu, Monica Chelliah, Owen Taylor

https://bugzilla.gnome.org/show_bug.cgi?id=661225

js/misc/util.js

@@ -18,7 +18,7 @@ const _urlRegexp = new RegExp(
     '(^|' + _leadingJunk + ')' +
     '(' +
         '(?:' +
-            '[a-z][\\w-]+://' +                   // scheme://
+            '(?:http|https|ftp)://' +             // scheme://
             '|' +
             'www\\d{0,3}[.]' +                    // www.
             '|' +