Only empty the message list DOM on reload

We are seeing error on CUSTOMER4 when clearing the DOM on reload. So
now we will only clear the message list.

(imported from commit f5d8d7d36cd1018f7def73ff9eda414387fcec5c)

static/js/reload.js

@@ -140,8 +140,8 @@ function cleanup_before_reload() {
         // Abort all pending ajax requests`
         channel.abort_all();
 
-        // Free all of the DOM
-        $("html").empty();
+        // Free all the DOM in the main_div
+        $("#main_div").empty();
 
         // Now that the DOM is empty our beforeunload callback has been
         // removed.