Adds an API endpoint for accessing read receipts for other users, as
well as a modal UI for displaying that information.
Enables the previously merged privacy settings UI for managing whether
a user makes read receipts data available to other users.
Documentation is pending, and we'll likely want to link to the
documentation with help_settings_link once it is complete.
Fixes#3618.
Co-authored-by: Tim Abbott <tabbott@zulip.com>
Dinesh