recent-topics: Add module.

Add methods to extract recent topics from received messages.
Process new messages as they are received.
Use new messages received from the server to extract recent_topics.
Node tests added.

static/js/recent_topics.js

@@ -0,0 +1,48 @@
+const topics = new Map(); // Key is stream-id:topic.
+
+exports.process_messages = function (messages) {
+    for (const msg of messages) {
+        exports.process_message(msg);
+    }
+};
+
+exports.process_message = function (msg) {
+    if (msg.type !== 'stream') {
+        return false;
+    }
+    // Initialize topic data
+    const key = msg.stream_id + ':' + msg.topic;
+    if (!topics.has(key)) {
+        topics.set(key, {
+            last_msg_id: -1,
+            starred: new Set(),
+            participated: false,
+            muted: false,
+        });
+    }
+    // Update topic data
+    const is_ours = people.is_my_user_id(msg.sender_id);
+    const topic_data = topics.get(key);
+    if (topic_data.last_msg_id < msg.id) {
+        topic_data.last_msg_id = msg.id;
+    }
+    if (msg.starred) {
+        topic_data.starred.add(msg.id);
+    }
+    topic_data.participated = is_ours || topic_data.participated;
+    topic_data.muted = topic_data.muted || muting.is_topic_muted(msg.stream_id, msg.topic);
+    return true;
+};
+
+function get_sorted_topics() {
+    // Sort all recent topics by last message time.
+    return new Map(Array.from(topics.entries()).sort(function (a, b) {
+        return  b[1].last_msg_id -  a[1].last_msg_id;
+    }));
+}
+
+exports.get = function () {
+    return get_sorted_topics();
+};
+
+window.recent_topics = exports;