hotspots: Add backend support for tutorial hotspots.

This commit adds the backend support for a new style of tutorial which
allows for highlighting of multiple areas of the page with hotspots that
disappear when clicked by the user.

static/js/server_events.js

@@ -23,6 +23,11 @@ function dispatch_normal_event(event) {
         admin.update_default_streams_table();
         break;
 
+    case 'hotspots':
+        hotspots.show(event.hotspots);
+        page_params.hotspots = event.hotspots;
+        break;
+
     case 'muted_topics':
         muting_ui.handle_updates(event.muted_topics);
         break;