This fix simplifies how we re-render topic lists when we
re-narrow or zoom out from a topic list.
* The topic_list.zoom_out() no longer gets called as
part of re-narrowing, and we eliminate the clear_topics
option.
* For all situations where we narrow to a filter that does
not have a topic, we simply call the new function
clear_topics().
* The stream_list code no longer calls remove_expanded_topics()
in cases where the new narrow has a topic. This allows us
to optimize away scroll/flicker churn a little more easily.
As part of this, we rename maybe_activate_stream_item() to
update_stream_sidebar_for_narrow(), since the function clears
stuff as well as turning stuff on.
fad024ebda