recent_senders: Rename process_message_for_senders.

Since it only processes stream messages, change its name to
`process_stream_message`.

static/js/message_helper.js

@@ -52,7 +52,7 @@ export function process_new_message(message) {
                 message_id: message.id,
             });
 
-            recent_senders.process_message_for_senders(message);
+            recent_senders.process_stream_message(message);
             message_user_ids.add_user_id(message.sender_id);
             break;