don't include file preview when editing syllabus body

test plan:
* create a syllabus description that references a
 file that can be previewed in-line
* save the syllabus and refresh page
* while viewing the syllabus preview the link in-line
* click to edit the syllabus (while the preview is open)
* confirm that the file preview html is not included
 in the editor html

fixes #CNVS-5976

Change-Id: I2ecbdc69b1df5c9f012509b0f6fa2109361493aa
Reviewed-on: https://gerrit.instructure.com/20880
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>

app/coffeescripts/behaviors/SyllabusBehaviors.coffee

@@ -163,13 +163,15 @@ define [
     $course_syllabus = $('#course_syllabus')
     $course_syllabus_details = $('#course_syllabus_details')
 
+    $course_syllabus.data('syllabus_body', ENV.SYLLABUS_BODY)
+
     wikiSidebar and wikiSidebar.init()
     $edit_course_syllabus_form.on 'edit', ->
       $edit_course_syllabus_form.show()
       $course_syllabus.hide()
       $course_syllabus_details.hide()
       $course_syllabus_body.editorBox()
-      $course_syllabus_body.editorBox 'set_code', $course_syllabus.html()
+      $course_syllabus_body.editorBox 'set_code', $course_syllabus.data('syllabus_body')
       if wikiSidebar
         wikiSidebar.attachToEditor $course_syllabus_body
         wikiSidebar.show()
@@ -210,6 +212,7 @@ define [
 
       success: (data) ->
         $course_syllabus.loadingImage('remove').html data.course.syllabus_body
+        $course_syllabus.data('syllabus_body', data.course.syllabus_body)
         $course_syllabus_details.hide()
 
       error: (data) ->

app/views/assignments/_syllabus_content.html.erb

@@ -1,4 +1,5 @@
 <% js_env :CAN_READ => can_do(@context, @current_user, :read) %>
+<% js_env :SYLLABUS_BODY => user_content(@syllabus_body) %>
 <% @active_tab = "syllabus" %>
 <% course_home ||= false %>
 <% if !course_home %>