Use topic_data.js for topic typeaheads.

This replaces some old code with calls to topic_data.js.

Now our topic typeahead uses the same data as our
sidebar, stream suggestions, and the "n" key, so any
future improvements to that data will benefit all
features the same.

This is an important piece of #9857.

frontend_tests/node_tests/general.js

@@ -89,10 +89,8 @@ const messages = {
 const noop = () => undefined;
 
 set_global('alert_words', {});
-set_global('composebox_typeahead', {});
 
 alert_words.process_message = noop;
-composebox_typeahead.add_topic = noop;
 
 // We can also bring in real code:
 zrequire('recent_senders');