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