feat: Shows the last activity, created at timestamp in the same row (#6267)

app/views/api/v1/conversations/partials/_conversation.json.jbuilder

@@ -37,6 +37,7 @@ json.labels conversation.label_list
 json.muted conversation.muted?
 json.snoozed_until conversation.snoozed_until
 json.status conversation.status
+json.created_at conversation.created_at.to_i
 json.timestamp conversation.last_activity_at.to_i
 json.first_reply_created_at conversation.first_reply_created_at.to_i
 json.unread_count conversation.unread_incoming_messages.count