added database driven system configurations with admin screen (#647)

src/Configuration/ConfigLoaderInterface.php

@@ -0,0 +1,21 @@
+<?php
+
+/*
+ * This file is part of the Kimai time-tracking app.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace App\Configuration;
+
+use App\Entity\Configuration;
+
+interface ConfigLoaderInterface
+{
+    /**
+     * @param null|string $prefix
+     * @return Configuration[]
+     */
+    public function getConfiguration(?string $prefix = null): array;
+}