[FIX] website_slides: fullscreen and previous/next

This solves 2 issues:

1) When the slide is set to fullscreen, on Firefox, the image was
stretched and the aspect ratio not respected. This is the default
behavior of Firefox for a canvas. By choosing the parentNode instead (a
div tag which only contains the canvas), the aspect ratio is respected.

2) Arrow key up/down has now the same behavior than arrow key
left/right.

opw-650174

addons/website_slides/static/lib/pdfslidesviewer/PDFSlidesViewer.js

@@ -146,7 +146,7 @@ var PDFSlidesViewer = (function(){
     }
 
     PDFSlidesViewer.prototype.toggleFullScreen = function(){
-        var el = this.canvas;
+        var el = this.canvas.parentNode;
 
         var isFullscreenAvailable = document.fullscreenEnabled || document.mozFullScreenEnabled || document.webkitFullscreenEnabled || document.msFullscreenEnabled || false;
         if(isFullscreenAvailable){ // Full screen supported

addons/website_slides/static/src/js/slides_embed.js

@@ -129,10 +129,10 @@ $(document).ready(function () {
 
         // switching slide with keyboard
         $(document).keydown(function (ev) {
-            if (ev.keyCode === 37) {
+            if (ev.keyCode === 37 || ev.keyCode === 38) {
                 embedded_viewer.previous();
             }
-            if (ev.keyCode === 39) {
+            if (ev.keyCode === 39 || ev.keyCode === 40) {
                  embedded_viewer.next();
             }
         });