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
There is a little bit problem here but, from my point of view, it's a problem in the `getJSON()` utility. Considering the current implementation, a `404 - not found` response resolves the promise with `null`. A malformed JSON will resolve the promise with `null` as well.

So, to receive a null is semantically the same as rejecting the error and the rejection code should be run in case of `json === null`. I would prefer to not guarding against this but changing the `getJSON()` implementation. I'll file a bug.