Version: warn users when deactivating a version or changing its slug

[stsewd]

When a version is deactivated or its slug is changed (and the version is active) we delete its documentation. That's an irreversible action, we should warn users about that with a pop-up. We already mention this in a warning message next to the form, but having a pop-up would be more explicit.

[humitos]

Adding some context here #561 (comment)