Pluggable storage backends: first steps

- add backend related columns in photo table
- add system resource storage class
- add code to load image data from backend class
- return "nosign" image as photo meta with SystemResource backend

src/Model/Storage/SystemResource.php

@@ -0,0 +1,39 @@
+<?php
+/**
+ * @file src/Model/Storage/SystemStorage.php
+ */
+
+namespace Friendica\Model\Storage;
+
+/**
+ * @brief System resource storage class
+ *
+ * This class is used to load system resources, like images.
+ * Is not itended to be selectable by admins as default storage class.
+ */
+class SystemResource
+{
+	// Valid folders to look for resources
+	const VALID_FOLDERS = [ "images" ];
+
+	/**
+	 * @brief get data
+	 *
+	 * @param string  $resourceid
+	 *
+	 * @return string
+	 */
+	static function get($filename)
+	{
+		$folder = dirname($filename);
+		if (!in_array($folder, self::VALID_FOLDERS)) return "";
+		if (!file_exists($filename)) return "";
+		return file_get_contents($filename);
+	}
+
+	static function put($filename, $data)
+	{
+		throw new \BadMethodCallException();
+	}
+}
+