Feature Request: configure automatic saving

[cb0s]

As described by others (for instance in #263 ), sometimes saving whiteboards fails for some reason. Apart from manually triggering "Save file", it would also help a lot imo, to create an option in the configuration of the Whiteboard Server to "Auto-Save" after a certain amount of time when the file was edited. This could drastically improve user experience as crashes or internet outage/internet instability, wouldn't lead to loosing too much data.