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 implementation (pulling data from the DOM and doing all the work) made this quite difficult. Setting the hash works as expected.