Replace a few forgotten obsolete helpers

- Apparently, I forgot that `array_flatten` comes from Laravel. :)
- When I did this previously, I did not search the views directory.

src/Extension/Extension.php

@@ -267,7 +267,7 @@ class Extension implements Arrayable
             $extenders = [$extenders];
         }
 
-        return array_flatten($extenders);
+        return Arr::flatten($extenders);
     }
 
     /**

src/Foundation/Site.php

@@ -9,6 +9,7 @@
 
 namespace Flarum\Foundation;
 
+use Illuminate\Support\Arr;
 use RuntimeException;
 
 class Site
@@ -62,6 +63,6 @@ class Site
             return [];
         }
 
-        return array_flatten($extenders);
+        return Arr::flatten($extenders);
     }
 }

src/Frontend/Content/Assets.php

@@ -13,6 +13,7 @@ use Flarum\Foundation\Application;
 use Flarum\Frontend\Compiler\CompilerInterface;
 use Flarum\Frontend\Document;
 use Illuminate\Contracts\Container\Container;
+use Illuminate\Support\Arr;
 use Psr\Http\Message\ServerRequestInterface as Request;
 
 class Assets
@@ -48,7 +49,7 @@ class Assets
         ];
 
         if ($this->app->inDebugMode()) {
-            $this->commit(array_flatten($compilers));
+            $this->commit(Arr::flatten($compilers));
         }
 
         $document->js = array_merge($document->js, $this->getUrls($compilers['js']));