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.

tools/test-js-with-node

@@ -49,6 +49,7 @@ enforce_fully_covered = {
     'static/js/search_suggestion.js',
     'static/js/stream_events.js',
     'static/js/stream_sort.js',
+    'static/js/topic_data.js',
     'static/js/topic_generator.js',
     'static/js/typeahead_helper.js',
     'static/js/typing_data.js',