message_list: Replace buggy rerender_the_whole_thing.

As it turns out, our rerender_the_whole_thing function (used whenever
we were adding messages and discovered that the resulting message list
would be out-of-order) was just broken and scrolled the browser to a
random location.

This caused two user-facing bugs:

* On very fast networks, if two users sent messages at very close to
  the same time, we could end up with out-of-order message deliveries,
  triggering this code path, which was intended to silently correct
  the situation, but failed.

* In some narrows to streams with muted topics in the history but some
  recent traffic, the user's browser-cached history might have some
  gaps that mean the server fetch we do after narrowing discovers the
  history is out-of-order, again triggering the
  rerender_the_whole_thing code path.

The fix is to just remove that function, adding a new option to the
well-tested rerender_preserving_scrolltop (which has explicit logic to
preserve the scroll position) instead.

Fixes #12067.  Likely also fixes #12498.

static/js/message_list.js

@@ -57,7 +57,7 @@ exports.MessageList.prototype = {
         var render_info;
 
         if (interior_messages.length > 0) {
-            self.view.rerender_the_whole_thing();
+            self.view.rerender_preserving_scrolltop(true);
             return true;
         }
         if (top_messages.length > 0) {