(core) A few improvements to CSS on mobile.

Summary:
- Adjust ExampleCard css to fit on x-small screens, including its close button,
  so that it may be closed.
- In TopBar on small screens, hide Notifications button and default action for
  Share; adjust spacing.
- Add overflow: hidden to PagePanels, to make sure it doesn't slide off of
  visible area.

Test Plan: Tested on iOS.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D2741

app/client/ui/PagePanels.ts

@@ -193,6 +193,7 @@ const cssPageContainer = styled(cssHBox, `
   position: absolute;
   isolation: isolate; /* Create a new stacking context */
   z-index: 0; /* As of March 2019, isolation does not have Edge support, so force one with z-index */
+  overflow: hidden;
   top: 0;
   left: 0;
   right: 0;
@@ -210,6 +211,7 @@ const cssPageContainer = styled(cssHBox, `
     }
   }
 `);
+
 export const cssLeftPane = styled(cssVBox, `
   position: relative;
   background-color: ${colors.lightGrey};