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/floating-header.js

@@ -0,0 +1,73 @@
+/* eslint-env browser */
+
+/**
+ * Floating header
+ * Used on invividual post pages, displays a sticky header with progress indicator
+ *
+ * This JS is automatically applied for any template where you use the
+ * {{> floating-header}} partial
+ */
+
+(function (window, document) {
+    // set up Casper as a global object
+    if (!window.Casper) {
+        window.Casper = {};
+    }
+
+    window.Casper.floatingHeader = function floatingHeader() {
+        // NOTE: Scroll performance is poor in Safari
+        // - this appears to be due to the events firing much more slowly in Safari.
+        //   Dropping the scroll event and using only a raf loop results in smoother
+        //   scrolling but continuous processing even when not scrolling
+
+        var progressBar = document.querySelector('#reading-progress');
+        var header = document.querySelector('.floating-header');
+        var title = document.querySelector('.post-full-title');
+
+        var lastScrollY = window.scrollY;
+        var lastWindowHeight = window.innerHeight;
+        var lastDocumentHeight = document.body.clientHeight;
+        var ticking = false;
+
+        function onScroll() {
+            lastScrollY = window.scrollY;
+            requestTick();
+        }
+
+        function onResize() {
+            lastWindowHeight = window.innerHeight;
+            lastDocumentHeight = document.body.clientHeight;
+            requestTick();
+        }
+
+        function requestTick() {
+            if (!ticking) {
+                requestAnimationFrame(update);
+            }
+            ticking = true;
+        }
+
+        function update() {
+            var trigger = title.getBoundingClientRect().top + window.scrollY;
+            var triggerOffset = title.offsetHeight + 35;
+            var progressMax = lastDocumentHeight - lastWindowHeight;
+
+            // show/hide floating header
+            if (lastScrollY >= trigger + triggerOffset) {
+                header.classList.add('floating-active');
+            } else {
+                header.classList.remove('floating-active');
+            }
+
+            progressBar.setAttribute('max', progressMax);
+            progressBar.setAttribute('value', lastScrollY);
+
+            ticking = false;
+        }
+
+        window.addEventListener('scroll', onScroll, {passive: true});
+        window.addEventListener('resize', onResize, false);
+
+        update();
+    };
+})(window, document);