settings: Add realm setting for Zulip communities directory.

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>

static/js/server_events_dispatch.js

@@ -224,6 +224,7 @@ export function dispatch_normal_event(event) {
                 video_chat_provider: compose.update_video_chat_button_display,
                 giphy_rating: giphy.update_giphy_rating,
                 waiting_period_threshold: noop,
+                want_advertise_in_communities_directory: noop,
                 wildcard_mention_policy: noop,
             };
             switch (event.op) {