(A11y severity 3) Set focus appropriately for sharing

[sathomas]

• When a user makes or cancels changes for the privacy and sharing options, keyboard focus is lost and thrown back to the top of the modal. While this isn’t a severe issue for such a small modal like this, it still creates additional effort and navigation for users. Set keyboard focus to a logical location for each item category when making changes (e.g., the "Change" and "Add more" buttons).
• Successfully sharing a document, as well as changing user access, throws keyboard focus back to the top of the page. Ensure that keyboard focus is set to a logical location on the page when modals close.