message view: Refactor top loading indicators.

This commit makes the `loading_older_messages_indicator` similar
to the `loading_newer_messages_indicator`.
Now all the decisions about whether to show a loading indicator
will be made from the `fetch_status` API. We still hide the
indicators everytime the view is changed, as explained in the
previous commit.

static/js/message_scroll.js

@@ -63,8 +63,6 @@ exports.scroll_finished = function () {
     if (message_viewport.at_top()) {
         message_fetch.maybe_load_older_messages({
             msg_list: current_msg_list,
-            show_loading: exports.show_loading_older,
-            hide_loading: exports.hide_loading_older,
         });
     }