Fix overlap issue with Sidebar and Dismissable banner (#1077)

* Increase zIndex of Sidebar to be on top of the page

As of now, the Sidebar is being overlapped by Dismissable banner component

* Unset z-index of Dismissable Banner

It inherits a z-index of 1400 from Material UI's snackbar component

packages/core/src/components/DismissableBanner/DismissableBanner.tsx

@@ -31,6 +31,7 @@ const useStyles = makeStyles((theme: Theme) => ({
     marginTop: -theme.spacing(3),
     display: 'flex',
     flexFlow: 'row nowrap',
+    zIndex: 'unset',
   },
   icon: {
     fontSize: 20,