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

.gitattributes

@@ -1,6 +1,8 @@
 .gitattributes export-ignore
 .gitignore export-ignore
 .gitmodules export-ignore
+.github export-ignore
+.travis export-ignore
 .travis.yml export-ignore
 .editorconfig export-ignore
 .styleci.yml export-ignore
@@ -8,4 +10,4 @@
 phpunit.xml export-ignore
 tests export-ignore
 
-js/*/dist/*.js -diff
+js/dist/* -diff