Extract topic_data.js.

This new module tracks the recent topic names for any given
stream.

The code was pulled over almost verbatim from stream_data.js,
with minor renames to the function names.

We introduced a minor one-line function called stream_has_topics.

static/js/topic_data.js

@@ -0,0 +1,70 @@
+var topic_data = (function () {
+
+var exports = {};
+
+var recent_topics = new Dict(); // stream_id -> array of objects
+
+exports.stream_has_topics = function (stream_id) {
+    return recent_topics.has(stream_id);
+};
+
+exports.process_message = function (message, remove_message) {
+    var current_timestamp = 0;
+    var count = 0;
+    var stream_id = message.stream_id;
+    var canon_topic = message.subject.toLowerCase();
+
+    var recents = recent_topics.get(stream_id) || [];
+
+    recents = _.filter(recents, function (item) {
+        var is_duplicate = (
+            item.name.toLowerCase() === canon_topic);
+        if (is_duplicate) {
+            current_timestamp = item.timestamp;
+            count = item.count;
+        }
+        return !is_duplicate;
+    });
+
+    if (remove_message !== undefined) {
+        count = count - 1;
+    } else {
+        count = count + 1;
+    }
+
+    if (count !== 0) {
+        recents.push({name: message.subject,
+                      count: count,
+                      timestamp: Math.max(message.timestamp, current_timestamp)});
+    }
+
+    recents.sort(function (a, b) {
+        return b.timestamp - a.timestamp;
+    });
+
+    recent_topics.set(stream_id, recents);
+};
+
+exports.get_recent_names = function (stream_id) {
+    var topic_objs = recent_topics.get(stream_id);
+
+    if (!topic_objs) {
+        return [];
+    }
+
+    return _.map(topic_objs, function (obj) {
+        return obj.name;
+    });
+};
+
+exports.populate_for_tests = function (stream_map) {
+    // This is only used by tests.
+    recent_topics = Dict.from(stream_map);
+};
+
+return exports;
+
+}());
+if (typeof module !== 'undefined') {
+    module.exports = topic_data;
+}