Defer processing of get_old_messages results during tutorial.

Previously we would just discard the results of get_old_messages,
which meant that any messages sent either while you were doing the
tutorial or that you started out with (as in the case of the CUSTOMER3
experiment) would be lost until you reloaded.

(imported from commit f5280c091ab6ed7c2af6eb8fe49c0fa6b997ac97)

static/js/tutorial.js

@@ -3,6 +3,7 @@ var tutorial = (function () {
 var exports = {};
 var is_running = false;
 var event_handlers = {};
+var deferred_work = [];
 
 // We'll temporarily set stream colors for the streams we use in the demo
 // tutorial messages.
@@ -223,6 +224,10 @@ function create_and_show_popover(target_div, placement, title, content_template)
     $(".popover").css("z-index", 20001);
 }
 
+exports.defer = function (callback) {
+    deferred_work.push(callback);
+};
+
 function finale() {
     var finale_modal = $("#tutorial-finale");
     $(".screen").css({opacity: 0.0});
@@ -245,6 +250,10 @@ function finale() {
     util.show_first_run_message();
     current_msg_list.rerender();
     enable_event_handlers();
+    _.each(deferred_work, function (callback) {
+        callback();
+    });
+    deferred_work = [];
 }
 
 function reply() {
@@ -397,9 +406,6 @@ exports.start = function () {
     }
     narrow.deactivate();
 
-    // If you somehow have messages, temporarily remove them from the visible
-    // feed.
-    current_msg_list.clear();
     // Set temporarly colors for the streams used in the tutorial.
     real_stream_info = subs.stream_info();
     subs.stream_info(tutorial_stream_info);