Concatenated all JS into a single file (#624)

no issue

- moved large inline JS from templates into separate JS files
  - floating header
  - gallery card support
- use `gulp-concat` to concatenate all JS files into a single `built/casper.js` file
- reduces external JS file requests from 3 (jquery, infinite-scroll.js, jquery.fitvids.js) down to 2 (jquery, casper.js) and reduces page size by removing repeated inline code

assets/js/gallery-card.js

@@ -0,0 +1,24 @@
+/* eslint-env browser */
+
+/**
+ * Gallery card support
+ * Used on any individual post/page
+ *
+ * Detects when a gallery card has been used and applies sizing to make sure
+ * the display matches what is seen in the editor.
+ */
+
+(function (window, document) {
+    var resizeImagesInGalleries = function resizeImagesInGalleries() {
+        var images = document.querySelectorAll('.kg-gallery-image img');
+        images.forEach(function (image) {
+            var container = image.closest('.kg-gallery-image');
+            var width = image.attributes.width.value;
+            var height = image.attributes.height.value;
+            var ratio = width / height;
+            container.style.flex = ratio + ' 1 0%';
+        });
+    };
+
+    document.addEventListener('DOMContentLoaded', resizeImagesInGalleries);
+})(window, document);