Reports zoom level and period persists app restart. closes #415

Current zoom level and period of reports are saved on settings and are used to initiate the reports page every time the user enters on it. #415 
Could store the enum as `int` instead of `string`. `return (ZoomLevel)intValue;`. Storing it as a string has the downside of when we should rename any of the enum values it will start crash and burning (unless you handle an exception which Enum.Parse probably throws).