Change API to use PSR-7 style requests and responses

This required some interface changes (mostly changing Laravel's or
Symfony's request and response classes to those of Zend's Diactoros.
Some smaller changes to the execution flow in a few of the abstract
action base classes, but nothing substantial.

Note: The request and response classes are immutable, so we usually
need to return new instances after modifying the old ones.

src/Api/ApiServiceProvider.php

@@ -1,7 +1,7 @@
 <?php namespace Flarum\Api;
 
+use Flarum\Http\Router;
 use Illuminate\Support\ServiceProvider;
-use Flarum\Api\Serializers\BaseSerializer;
 
 class ApiServiceProvider extends ServiceProvider
 {
@@ -17,6 +17,8 @@ class ApiServiceProvider extends ServiceProvider
             'Flarum\Api\ExceptionHandler'
         );
 
+        $this->app->singleton('Flarum\Http\Router', function() { return new Router(); });
+
         include __DIR__.'/routes.php';
     }