Dashboard versions history

[vkaverin]

Sometimes it's necessary to recover a previous version of a dashboard: current version may be broken or something was lost by mistake during editing.
Also, t would be also good to have a preview of the dashboard version before restoring it.
And it would also be nice if there's a way to see the difference between versions (but probably it's possible only for JSON-like representation of the dashboard, not sure if it's possible to show the difference visually).