add backup/restore REST API

sftpgo.json

@@ -45,6 +45,7 @@
     "bind_port": 8080,
     "bind_address": "127.0.0.1",
     "templates_path": "templates",
-    "static_files_path": "static"
+    "static_files_path": "static",
+    "backups_path": "backups"
   }
 }