Webpack (#14)

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/lock/less/forum.less

@@ -1,6 +1,3 @@
-.Badge--locked {
-  background: #888;
-}
 .DiscussionLockedPost {
   .EventPost-icon,
   .EventPost-info,