Implement proper update process

If the version in the settings table mismatches the code version, then we return a 503 error for all requests coming through index.php and api.php, while admin.php serves up a form prompting for the database password which will run outstanding migrations.

src/Update/Controller/UpdateController.php

@@ -0,0 +1,68 @@
+<?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\Update\Controller;
+
+use Exception;
+use Flarum\Foundation\Application;
+use Flarum\Http\Controller\ControllerInterface;
+use Flarum\Update\Console\MigrateCommand;
+use Illuminate\Contracts\Bus\Dispatcher;
+use Psr\Http\Message\ServerRequestInterface as Request;
+use Symfony\Component\Console\Input\StringInput;
+use Symfony\Component\Console\Output\StreamOutput;
+use Zend\Diactoros\Response;
+use Zend\Diactoros\Response\HtmlResponse;
+
+class UpdateController implements ControllerInterface
+{
+    protected $command;
+
+    /**
+     * @var Application
+     */
+    protected $app;
+
+    /**
+     * @param MigrateCommand $command
+     * @param Application $app
+     */
+    public function __construct(MigrateCommand $command, Application $app)
+    {
+        $this->command = $command;
+        $this->app = $app;
+    }
+
+    /**
+     * @param Request $request
+     * @param array $routeParams
+     * @return \Psr\Http\Message\ResponseInterface
+     */
+    public function handle(Request $request, array $routeParams = [])
+    {
+        $input = $request->getParsedBody();
+
+        if (array_get($input, 'databasePassword') !== $this->app->config('database.password')) {
+            return new HtmlResponse('Incorrect database password.', 500);
+        }
+
+        $body = fopen('php://temp', 'wb+');
+        $input = new StringInput('');
+        $output = new StreamOutput($body);
+
+        try {
+            $this->command->run($input, $output);
+        } catch (Exception $e) {
+            return new HtmlResponse($e->getMessage(), 500);
+        }
+
+        return new Response($body, 200);
+    }
+}