hotkeys: Add lightbox image feed with controls.

This adds an image feed that you can scroll through with hotkeys
in the lightbox.

The left and right arrow keys along with the left and right arrows
will go to the prev/next image, and clicking on an image will also
take a user to that image.

static/js/modals.js

@@ -21,6 +21,7 @@ var modals = (function () {
 
             lightbox: function () {
                 $(".player-container iframe").remove();
+                lightbox.is_open = false;
                 document.activeElement.blur();
             },