We were previously calling sort_narrow_list after each stream was was
added. Because it is linear in the current length of the sidebar
list, we were doing quadratic work on page load. When we enabled the
streams sidebar on the MIT realm, this became problematic because of
the number of subscriptions Zephyr users have.
(imported from commit d60ddc638f0a81fbce08eecd6671e9ea6ca38515)