Fixed double scrollbar bug

Ref: https://github.com/TryGhost/Casper/issues/850

Portal has a bug that the scrollbars for the popup wrapper are always
visible which results in double scrollbars for certian viewports.
This commit hides the wrapper's scrollbars, so only the site scrollbars
will be visible (if the content requires it).

ghost/portal/src/components/Frame.styles.js

@@ -235,6 +235,12 @@ const FrameStyles = `
         height: 100%;
         max-height: 100vh;
         overflow: scroll;
+        -ms-overflow-style: none;
+        scrollbar-width: none;
+    }
+
+    .gh-portal-popup-wrapper::-webkit-scrollbar {
+        display: none;
     }
 
     .gh-portal-popup-container {