back button returns to app from screenshots view (bug 972513)

https://bugzilla.mozilla.org/show_bug.cgi?id=972513

I spent a good amount of time trying to get the screenshots to be a proper view and linkable. I would still prefer that but the composite view (app detail view with screenshots layered on top) and the current [lightbox.js](hearth/media/js/lightbox.js) implementation (pulling data from the DOM and doing all the work) made this quite difficult. Setting the hash works as expected.