Refactor of micropub request handling

Trying to organise the code better. It now temporarily doesn’t support
update requests. Thought the spec defines them as SHOULD features and
not MUST features. So safe for now :)

bootstrap/providers.php

@@ -3,4 +3,5 @@
 return [
     App\Providers\AppServiceProvider::class,
     App\Providers\HorizonServiceProvider::class,
+    App\Providers\MicropubServiceProvider::class,
 ];