Webpack (#17)

See https://github.com/flarum/core/pull/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"
* Restructure `less` directory

extensions/subscriptions/less/forum.less

@@ -0,0 +1,29 @@
+.Badge--following {
+  background: #ffea7b;
+  color: #de8e00;
+}
+.Badge--ignoring {
+  background: #aaa;
+}
+.SubscriptionMenu-button--follow {
+  .Button--color(#de8e00, #fff2ae);
+  // TODO: dark mode
+}
+.SubscriptionMenu .Dropdown-menu {
+  min-width: 260px;
+}
+.SubscriptionMenuItem-label {
+  padding-left: 25px;
+  display: block;
+  white-space: normal;
+
+  & strong {
+    display: block;
+  }
+}
+.SubscriptionMenuItem-description {
+  display: block;
+  color: @muted-color;
+  font-size: 12px;
+  margin-top: 3px;
+}