Bug 1062806 - Use the new shared helper for loading JSON files in the co..

...st control app

https://bugzilla.mozilla.org/show_bug.cgi?id=1062806
In view of [comment 7 in bug 1062806](https://bugzilla.mozilla.org/show_bug.cgi?id=1062806#c7), let's file a bug for changing the `getJSON()` implementation, add the `null` check and leave a `TODO` comment referencing the bug. Sync with @gabrielesvelto to get this done, please.