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

less/common/mixins/header-background.less

@@ -0,0 +1,18 @@
+.header-background() {
+  background: fade(@header-bg, 98%);
+  position: fixed;
+  top: 0;
+  left: 0;
+  right: 0;
+  z-index: @zindex-header;
+  border-bottom: 1px solid @control-bg;
+  .translate3d(0, 0, 0);
+  .transition(~"box-shadow 0.2s, -webkit-transform 0.2s");
+
+  @media @phone {
+    height: @header-height-phone;
+  }
+  @media @tablet-up {
+    height: @header-height;
+  }
+}