Send an email when a server is marked as installed (#1213)

Co-authored-by: @stanjg

app/Contracts/Core/ReceivesEvents.php

@@ -0,0 +1,15 @@
+<?php
+
+namespace Pterodactyl\Contracts\Core;
+
+use Pterodactyl\Events\Event;
+
+interface ReceivesEvents
+{
+    /**
+     * Handles receiving an event from the application.
+     *
+     * @param \Pterodactyl\Events\Event $notification
+     */
+    public function handle(Event $notification): void;
+}