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

src/Configuration/SystemBundleConfiguration.php

@@ -0,0 +1,24 @@
+<?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;
+
+interface SystemBundleConfiguration
+{
+    /**
+     * @return string
+     */
+    public function getPrefix(): string;
+
+    /**
+     * @param string $key
+     * @return mixed
+     */
+    public function find(string $key);
+}