Deduplicated code for image expansion into the page overlay

It now also supports WEBP links, and is compatible with Google Cloud
Storage (which adds ?blablabla to links).

src/scripts/tutti/8_expand_image.js

@@ -0,0 +1,18 @@
+$(function() {
+    /* Expand images when their link points to a jpg/png/gif/webp */
+    var imgs = $('.expand-image-links a img')
+    .off('click')
+    .on('click', function(e){
+        var $img = $(this);
+        var href = $img.parent().attr('href');
+
+        if (href.match("\\.(jpg|png|gif|webp)\\W?")) {
+            var src = $img.attr('src');
+            var overlay_img = $('<img>').attr('src', src);
+            $('#page-overlay')
+                        .addClass('active')
+                        .html(overlay_img);
+            e.preventDefault();
+        }
+    });
+});