recent_topics: Add Private message to recent_topics.

This commit adds private messages to the Recent topics view, to make
it an all-encompassing overview of recent activity visible to the user.

We add a filter "Include PM" to toggle whether PMs should be shown in
recent topics.

Fixes #19449.

static/js/recent_topics_util.js

@@ -29,3 +29,15 @@ export function is_in_focus() {
 export function get_topic_key(stream_id, topic) {
     return stream_id + ":" + topic.toLowerCase();
 }
+
+export function get_key_from_message(msg) {
+    if (msg.type === "private") {
+        // The to_user_ids field on a private message object is a
+        // string containing the user IDs involved in the message in
+        // sorted order.
+        return msg.to_user_ids;
+    } else if (msg.type === "stream") {
+        return get_topic_key(msg.stream_id, msg.topic);
+    }
+    throw new Error(`Invalid message type ${msg.type}`);
+}