message_edit: Apply topic edit restrictions to "(no topic)" messages.

Previously, editing topic of "(no topic)" messages was allowed
irrespective of time limit or the "edit_topic_policy" setting.
Since we are working in the direction of having "no topic" messages
feel reasonable, this commit changes the code to not consider them
as a special case and topic editing restrictions apply to them as
well now like all other messages.

We still highlight the topic edit icon in recipient bar without
hovering for "no topic" messages, but it is only shown when user
has permission to edit topics.

api_docs/changelog.md

@@ -20,6 +20,11 @@ format used by the Zulip server that they are interacting with.
 
 ## Changes in Zulip 7.0
 
+**Feature level 172**
+
+* [`PATCH /messages/{message_id}`](/api/update-message): Topic editing
+  restrictions now apply to messages without a topic as well.
+
 **Feature level 171**:
 
 * [`POST /fetch_api_key`](/api/fetch-api-key),