Webpack (#1367)

* Replace gulp with webpack and npm scripts for JS compilation
* Set up Travis CI to commit compiled JS
* Restructure `js` directory; only one instance of npm, forum/admin are "submodules"
* Refactor JS initializers into Application subclasses
* Maintain partial compatibility API (importing from absolute paths) for extensions
* Remove minification responsibility from PHP asset compiler
* Restructure `less` directory

js/src/common/utils/SubtreeRetainer.js

@@ -0,0 +1,68 @@
+/**
+ * The `SubtreeRetainer` class represents a Mithril virtual DOM subtree. It
+ * keeps track of a number of pieces of data, allowing the subtree to be
+ * retained if none of them have changed.
+ *
+ * @example
+ * // constructor
+ * this.subtree = new SubtreeRetainer(
+ *   () => this.props.post.freshness,
+ *   () => this.showing
+ * );
+ * this.subtree.check(() => this.props.user.freshness);
+ *
+ * // view
+ * this.subtree.retain() || 'expensive expression'
+ *
+ * @see https://lhorie.github.io/mithril/mithril.html#persisting-dom-elements-across-route-changes
+ */
+export default class SubtreeRetainer {
+  /**
+   * @param {...callbacks} callbacks Functions returning data to keep track of.
+   */
+  constructor(...callbacks) {
+    this.callbacks = callbacks;
+    this.data = {};
+  }
+
+  /**
+   * Return a virtual DOM directive that will retain a subtree if no data has
+   * changed since the last check.
+   *
+   * @return {Object|false}
+   * @public
+   */
+  retain() {
+    let needsRebuild = false;
+
+    this.callbacks.forEach((callback, i) => {
+      const result = callback();
+
+      if (result !== this.data[i]) {
+        this.data[i] = result;
+        needsRebuild = true;
+      }
+    });
+
+    return needsRebuild ? false : {subtree: 'retain'};
+  }
+
+  /**
+   * Add another callback to be checked.
+   *
+   * @param {...Function} callbacks
+   * @public
+   */
+  check(...callbacks) {
+    this.callbacks = this.callbacks.concat(callbacks);
+  }
+
+  /**
+   * Invalidate the subtree, forcing it to be rerendered.
+   *
+   * @public
+   */
+  invalidate() {
+    this.data = {};
+  }
+}