Use Laravel's class-based Str and Arr helpers

Starting with version 5.9, the global funtions will be deprecated.

* https://laravel-news.com/laravel-5-8-deprecates-string-and-array-helpers
* https://github.com/laravel/framework/pull/26898

src/Discussion/Command/EditDiscussionHandler.php

@@ -17,6 +17,7 @@ use Flarum\Discussion\Event\Saving;
 use Flarum\Foundation\DispatchEventsTrait;
 use Flarum\User\AssertPermissionTrait;
 use Illuminate\Contracts\Events\Dispatcher;
+use Illuminate\Support\Arr;
 
 class EditDiscussionHandler
 {
@@ -54,7 +55,7 @@ class EditDiscussionHandler
     {
         $actor = $command->actor;
         $data = $command->data;
-        $attributes = array_get($data, 'attributes', []);
+        $attributes = Arr::get($data, 'attributes', []);
 
         $discussion = $this->discussions->findOrFail($command->discussionId, $actor);
 

src/Discussion/Command/StartDiscussionHandler.php

@@ -20,6 +20,7 @@ use Flarum\Post\Command\PostReply;
 use Flarum\User\AssertPermissionTrait;
 use Illuminate\Contracts\Bus\Dispatcher as BusDispatcher;
 use Illuminate\Contracts\Events\Dispatcher as EventDispatcher;
+use Illuminate\Support\Arr;
 
 class StartDiscussionHandler
 {
@@ -66,7 +67,7 @@ class StartDiscussionHandler
         // an opportunity to alter the discussion entity based on data in the
         // command they may have passed through in the controller.
         $discussion = Discussion::start(
-            array_get($data, 'attributes.title'),
+            Arr::get($data, 'attributes.title'),
             $actor
         );