Bundle unminified JS; minify via ClosureCompilerService when in production

Falls back to a less effective minification library if ClosureCompilerService errors or is unavailable. Minification takes a while (20 seconds or so), but it only happens when assets are modified. Still, this means enabling/disabling extensions is taking far too long. Possible solutions:

- Don't minify initially; set a process running in the background to do minification, and server unminified assets in the meantime.
- Refactor compiler to send each JS file to CCS individually, only if that particular file has been modified.

flarum/gulp has also been updated to no longer support uglification.

closes #582

framework/core/src/Http/Controller/AbstractClientController.php

@@ -187,7 +187,7 @@ abstract class AbstractClientController extends AbstractHtmlController
         $public = $this->getAssetDirectory();
 
         $assets = new AssetManager(
-            new JsCompiler($public, "$this->clientName.js"),
+            new JsCompiler($public, "$this->clientName.js", ! $this->app->config('debug')),
             new LessCompiler($public, "$this->clientName.css", $this->app->storagePath().'/less')
         );