Chat access notification

Features of this pull request:
- Removed old accessible-chat-notification system, which was poorly reviewed in end-user testing
- Removed obsolete comments referring to the old system
- Introduced a small notification sound (licensed under Creative Commons Zero) to alert users with screen readers that there is a new message

Abandoned features:
- Changing the accessibilityDescription of a given chat tab to <existing label> + "NEW MESSAGE" when a message is received, so a screen reader can indicate to the user which tabs contain unread messages. Abandoned because SuperTabNavigator behaves poorly when accessibility properties are changed.