users: Return true for admins for is_moderator.

This change is for consistency with how is_admin works.

API design discussion at https://chat.zulip.org/#narrow/channel/378-api-design/topic/Should.20is_moderator.20have.20admins

api_docs/changelog.md

@@ -20,6 +20,13 @@ format used by the Zulip server that they are interacting with.
 
 ## Changes in Zulip 11.0
 
+**Feature level 380**
+
+* [`POST /register`](/api/register-queue), [`GET
+  /events`](/api/get-events): The `is_moderator` convenience field now
+  is true for organization administrators, matching how `is_admin`
+  works for organization owners.
+
 **Feature level 379**
 
 * [`PATCH /messages/{message_id}`](/api/update-message): Added

api_docs/roles-and-permissions.md

@@ -36,7 +36,8 @@ by the [events API](/api/get-events).
 
 Note that [`POST /register`](/api/register-queue) also returns an
 `is_moderator` boolean property specifying whether the current user is
-an organization moderator.
+at least an organization moderator. The property will be true for admins
+and owners too.
 
 Additionally, user account data include an `is_billing_admin` property
 specifying whether the user is a billing administrator for the Zulip