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/admin/app.less

@@ -1,10 +0,0 @@
-@import "../lib/lib.less";
-
-@import "AdminNav.less";
-@import "DashboardPage.less";
-@import "BasicsPage.less";
-@import "PermissionsPage.less";
-@import "EditGroupModal.less";
-@import "ExtensionsPage.less";
-@import "AppearancePage.less";
-@import "MailPage.less";