Split up Site into several classes

Depending on the state of the Flarum installation (installed, not
installed, currently upgrading, maintenance mode), we should enable
different sets of service providers.

For example, during installation we should not resolve a setting
repository from the container. This new architecture lets us do so,
but we can easily (and cleanly) register a different implementation
during installation.

This should prevent problems such as #1370 in the future.

src/Foundation/AppInterface.php

@@ -0,0 +1,25 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Flarum\Foundation;
+
+interface AppInterface
+{
+    /**
+     * @return \Psr\Http\Server\RequestHandlerInterface
+     */
+    public function getRequestHandler();
+
+    /**
+     * @return \Symfony\Component\Console\Command\Command[]
+     */
+    public function getConsoleCommands();
+}