order exports in /courses/X/content_exports

fixes CNVS-14765

test plan:
 - have a course with multiple content exports
 - /courses/X/content_exports should display them in order
   by date, descending

Change-Id: I76356ab83b9631dd192dea18c331b425505a92d2
Reviewed-on: https://gerrit.instructure.com/39252
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: James Williams  <jamesw@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>

app/controllers/content_exports_controller.rb

@@ -27,7 +27,7 @@ class ContentExportsController < ApplicationController
   end
 
   def index
-    @exports = @context.content_exports_visible_to(@current_user).active.not_for_copy
+    @exports = @context.content_exports_visible_to(@current_user).active.not_for_copy.order('created_at DESC')
 
     @current_export_id = nil
     if export = @context.content_exports_visible_to(@current_user).running.first

spec/controllers/content_exports_controller_spec.rb

@@ -75,7 +75,7 @@ describe ContentExportsController do
           user_session(@teacher)
           get :index, course_id: @course.id
           response.should be_success
-          assigns(:exports).map(&:id).should == [@acx.id, @tcx.id, @tzx.id]
+          assigns(:exports).map(&:id).should =~ [@acx.id, @tcx.id, @tzx.id]
         end
       end