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/Formatter/Formatter.php

@@ -198,10 +198,6 @@ class Formatter
         $configurator->enableJavaScript();
         $configurator->javascript->exportMethods = ['preview'];
 
-        $minifier = $configurator->javascript->setMinifier(new MinifyMinifier);
-        $minifier->keepGoing = true;
-        $minifier->cacheDir = $this->cacheDir;
-
         return $configurator->finalize([
             'returnParser' => false,
             'returnRenderer' => false