Allow easier extensibility of page document

This allows extensions to:

- mutate the json api document sent with php documents
- override/interact with the Document

Right now extensions need to replace the complete Frontend class
in order to interact with the document. Let's abstract into ioc
so that advanced devs can interact with it.

src/Frontend/Frontend.php

@@ -49,7 +49,9 @@ class Frontend
     {
         $forumDocument = $this->getForumDocument($request);
 
-        $document = new Document($this->view, $forumDocument, $request);
+        $responseDocument = resolve('flarum.frontend.document');
+
+        $document = $responseDocument($forumDocument, $request);
 
         $this->populate($document, $request);
 

src/Frontend/FrontendServiceProvider.php

@@ -16,6 +16,8 @@ use Flarum\Http\UrlGenerator;
 use Flarum\Settings\SettingsRepositoryInterface;
 use Illuminate\Contracts\Container\Container;
 use Illuminate\Contracts\View\Factory as ViewFactory;
+use Illuminate\View\Factory;
+use Psr\Http\Message\ServerRequestInterface as Request;
 
 class FrontendServiceProvider extends AbstractServiceProvider
 {
@@ -57,6 +59,16 @@ class FrontendServiceProvider extends AbstractServiceProvider
                 return $frontend;
             };
         });
+
+        $this->container->singleton('flarum.frontend.document', function (Container $container) {
+            return function (array $apiDocument, Request $request) use ($container){
+                return new Document(
+                    $container->make(Factory::class),
+                    $apiDocument,
+                    $request
+                );
+            };
+        });
     }
 
     /**