message_fetch: Increase batch size for message backfill to 1000.

There's no advantage to doing a small batch size towards current here,
since latency isn't an issue at this point, and performance on the
server side generally favors larger batch sizes.

This also will make it significantly harder to start getting 429 rate
limiting errors when loading when far behind current.

static/js/message_fetch.js

@@ -178,7 +178,7 @@ util.execute_early(function () {
                 exports.load_old_messages({
                     anchor: latest_id.toFixed(),
                     num_before: 0,
-                    num_after: 400,
+                    num_after: 1000,
                     msg_list: home_msg_list,
                     cont: load_more,
                 });