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

src/Foundation/Application.php

@@ -110,6 +110,16 @@ class Application extends Container implements ApplicationContract
         return $this->bound('flarum.config');
     }
 
+    /**
+     * @param string $key
+     * @param mixed $default
+     * @return mixed
+     */
+    public function config($key, $default = null)
+    {
+        return array_get($this->make('flarum.config'), $key, $default);
+    }
+
     /**
      * Check if Flarum is in debug mode.
      *
@@ -117,7 +127,7 @@ class Application extends Container implements ApplicationContract
      */
     public function inDebugMode()
     {
-        return ! $this->isInstalled() || $this->make('flarum.config')['debug'];
+        return ! $this->isInstalled() || $this->config('debug');
     }
 
     /**