api: Add a new endpoint for message reporting.

This adds a new API endpoint that enables users to report messages for
review by admins or moderators. Reports will be sent to the
`moderate_request_channel`, so it must be configured for this feature to
be enabled.

Fixes part of #20047.

Co-authored-by: Adam Sah <140002+asah@users.noreply.github.com>

api_docs/changelog.md

@@ -20,6 +20,11 @@ format used by the Zulip server that they are interacting with.
 
 ## Changes in Zulip 11.0
 
+**Feature level 382**
+
+* `POST /message/{message_id}/report`: Added a new endpoint for submitting
+  a moderation request for a message.
+
 **Feature level 381**
 
 * [`POST /reminders`](/api/create-message-reminder): Added a new endpoint to

api_docs/include/rest-endpoints.md

@@ -18,6 +18,7 @@
 * [Mark messages in a channel as read](/api/mark-stream-as-read)
 * [Mark messages in a topic as read](/api/mark-topic-as-read)
 * [Get a message's read receipts](/api/get-read-receipts)
+* [Report a message](/api/report-message)
 
 #### Scheduled messages