Webpack (#33)

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/mentions/js/package.json

@@ -0,0 +1,14 @@
+{
+  "name": "@flarum/flarum-ext-mentions",
+  "version": "0.0.0",
+  "dependencies": {
+    "flarum-webpack-config": "^0.1.0-beta.8",
+    "textarea-caret": "^3.1.0",
+    "webpack": "^4.0.0",
+    "webpack-cli": "^3.0.7"
+  },
+  "scripts": {
+    "build": "webpack --mode production --progress",
+    "watch": "webpack --mode development --watch"
+  }
+}