Add label for session started

Fixes CNVS-17760

Test plan
 - Enable QLA
 - Take a quiz
 - There should be a "Session started" event

Change-Id: I39240ce040f8253b1fc60d6f9be5fae5802f98f2
Reviewed-on: https://gerrit.instructure.com/46642
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Ryan Taylor <rtaylor@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Brian Finney <bfinney@instructure.com>

client_apps/canvas_quizzes/apps/events/js/constants.js

@@ -1,4 +1,5 @@
 define({
+  EVT_SESSION_STARTED: 'session_started',
   EVT_PAGE_FOCUSED: 'page_focused',
   EVT_PAGE_BLURRED: 'page_blurred',
   EVT_QUESTION_VIEWED: 'question_viewed',
@@ -50,4 +51,4 @@ define({
   // Answer text longer than this will be truncated for questions of types
   // "essay" and other free-form input ones. This applies to the table view.
   MAX_VISIBLE_CHARS: 50
-});
+});

client_apps/canvas_quizzes/apps/events/js/views/event_stream/event.jsx

@@ -68,6 +68,10 @@ define(function(require) {
       var label;
 
       switch(event.type) {
+        case K.EVT_SESSION_STARTED:
+          description = I18n.t('session_started', 'Session started');
+        break;
+
         case K.EVT_QUESTION_ANSWERED:
           label = I18n.t('question_answered', {
             one: 'Answered question:',