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/Navigation.less

@@ -0,0 +1,60 @@
+.Navigation-back {
+  z-index: 3 !important; // z-index of an active .btn-group .btn is 2
+  border-radius: @border-radius !important;
+  .transition(border-radius 0.2s);
+  max-width: 150px;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  padding-left: 8px;
+  padding-right: 8px;
+
+  .icon {
+    font-size: 14px;
+  }
+}
+.Navigation-pin {
+  display: none;
+  opacity: 0;
+  margin-left: -5px !important;
+  border-radius: 0 @border-radius @border-radius 0;
+  .transition(~"opacity 0.2s, margin-left 0.2s");
+
+  .icon {
+    .rotate(45deg);
+  }
+}
+
+@media @desktop-hd {
+  .Navigation-pin {
+    display: block;
+  }
+  .hasPane.panePinned, .hasPane.paneShowing {
+    .Navigation-back {
+      border-radius: @border-radius 0 0 @border-radius !important;
+    }
+    .Navigation-pin {
+      opacity: 1;
+      margin-left: 1px !important;
+    }
+  }
+  .hasPane.panePinned .Navigation-pin .icon {
+    .rotate(0deg);
+  }
+}
+
+.Navigation-drawer.new {
+  position: relative;
+
+  &:after {
+    content: ' ';
+    display: block;
+    position: absolute;
+    background: @primary-color;
+    top: 10px;
+    right: 10px;
+    width: 14px;
+    height: 14px;
+    border-radius: 7px;
+    border: 2px solid @body-bg;
+  }
+}