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

partials/floating-header.hbs

@@ -26,3 +26,11 @@
         </div>
     </progress>
 </div>
+
+{{#contentFor "scripts"}}
+<script>
+    $(document).ready(function () {
+        Casper.floatingHeader();
+    })
+</script>
+{{/contentFor}}