use DisucssionTopic::ScopedToUser in CC::TopicResources#add_topics

fixes CNVS-21394

The content export tool is being updated so that it consders the
provided user's rights, so that the CC export can be used for the new
epub generation tool.

This PS employs DiscussionTopic::ScopedToUser to make sure that the user
only gets the discussion topics that they have the right to view.

test plan:
- Regression test for content export tool discussion topic functionality.

Change-Id: Id67bf1ed69c4b2066ff9d81678a5d61d52702978
Reviewed-on: https://gerrit.instructure.com/59365
Tested-by: Jenkins
Reviewed-by: Brian Finney <bfinney@instructure.com>
QA-Review: Deepeeca Soundarrajan <dsoundarrajan@instructure.com>
Product-Review: Cameron Sutter <csutter@instructure.com>

lib/cc/topic_resources.rb

@@ -17,16 +17,17 @@
 #
 module CC
   module TopicResources
-    
+
     def add_topics
-      @course.discussion_topics.active.each do |topic|
+      scope = @course.discussion_topics.active
+      DiscussionTopic::ScopedToUser.new(@course, @user, scope).scope.each do |topic|
         if topic.is_announcement
           next unless export_object?(topic, 'announcements')
         else
           next unless export_object?(topic) || export_object?(topic.assignment)
         end
 
-        title = topic.title rescue I18n.t('course_exports.unknown_titles.topic', "Unknown topic")
+        title = topic.title || I18n.t('course_exports.unknown_titles.topic', "Unknown topic")
 
         if topic.assignment && !topic.assignment.can_copy?(@user)
           add_error(I18n.t('course_exports.errors.topic_is_locked', "The topic \"%{title}\" could not be copied because it is locked.", :title => title))