Add user backend provider system

app/ServiceProvider/UserProvider.php

@@ -0,0 +1,35 @@
+<?php
+
+namespace Kanboard\ServiceProvider;
+
+use Kanboard\Core\User\UserManager;
+use Kanboard\User\DatabaseBackendUserProvider;
+use Pimple\Container;
+use Pimple\ServiceProviderInterface;
+
+/**
+ * User Provider
+ *
+ * @package Kanboard\ServiceProvider
+ * @author  Frederic Guillot
+ */
+class UserProvider implements ServiceProviderInterface
+{
+    /**
+     * Register providers
+     *
+     * @access public
+     * @param  \Pimple\Container $container
+     * @return \Pimple\Container
+     */
+    public function register(Container $container)
+    {
+        $container['userManager'] = new UserManager();
+
+        if (DB_USER_PROVIDER) {
+            $container['userManager']->register(new DatabaseBackendUserProvider($container));
+        }
+
+        return $container;
+    }
+}