left sidebar: Fix scrolling bugs with zoomed topics.

If you zoom into "more topics" for a stream that has
a LOT of topics, and then scroll down to the bottom,
and then zoom out by selecting "All messages" or
similar upper-left-sidebar options, we now try to scroll
the more recently active stream back into place after we scroll
out.

Before this change, it was possible for your lower left
sidebar to appear empty, as it would keep the
scroll offset from "more topics".

frontend_tests/node_tests/general.js

@@ -656,6 +656,7 @@ function make_topic_list_helper() {
     // care about the implementation details of topic_list for
     // now, just that it is invoked properly.
     topic_list.active_stream_id = () => undefined;
+    topic_list.get_stream_li = () => undefined;
 
     var topic_list_closed;
     topic_list.close = () => {