Changes the admin UI for the communities directory checkbox to use
the `realm_push_notifications_enabled` page param instead of the
`server_web_public_streams_enabled` page param.
Updates help center documentation about the communities directory
to have clearer information about how the setting works for
self-hosted communitites.
Adds `want_advertise_in_communities_directory` to the realm model
to track organizations that give permission to be listed on such
a site / directory on zulip.com.
Adds a checkbox to the organization profile admin for
organizations to give permission to be advertised in the
Zulip communities directory.
Adds a help center article about the Zulip communities directory
and uses a shared intro documentation file to create sections in
the articles on creating an organization profile and moderating
open organizations.
Co-authored-by: Alya Abbott <alya@zulip.com>
Lauryn Menard