warning the user if he tries to close the page (or navigate away)

[teolemon]

          The loss of changes is however a big deal @perierc 

As we have hasChanges I think there is a way to handle that by warning the user if he tries to close the page (or navigate away):
see https://developer.mozilla.org/en-US/docs/Web/API/Window/beforeunload_event

It might be in another PR though.

Originally posted by @alexgarel in #468 (comment)

[aapatil2004]

Can i work on this issue

[alexgarel]

Just to be precise, as reported on the PR, navigate away means:

1. either closing the tab and / or typing / clicking a new external url
2. either navigating to another page of the app.

While the first might be solved with beforeunload_event,
The second needs to be handled in React routing (because the page does not really change from the browser point of view).