Make sequence module use ajax_url properly

common/lib/xmodule/xmodule/js/src/sequence/display.coffee

@@ -4,7 +4,7 @@ class @Sequence
     @contents = @$('.seq_contents')
     @num_contents = @contents.length
     @id = @el.data('id')
-    @modx_url = @el.data('course_modx_root')
+    @ajaxUrl = @el.data('ajax-url')
     @initProgress()
     @bind()
     @render parseInt(@el.data('position'))
@@ -84,7 +84,7 @@ class @Sequence
     if @position != new_position
       if @position != undefined
         @mark_visited @position
-        modx_full_url = @modx_url + '/' + @id + '/goto_position'
+        modx_full_url = '#{@ajaxUrl}/goto_position'
         $.postWithPrefix modx_full_url, position: new_position
 
       # On Sequence change, fire custom event "sequence:change" on element.

common/lib/xmodule/xmodule/seq_module.py

@@ -98,6 +98,7 @@ class SequenceModule(SequenceFields, XModule):
                   'item_id': self.id,
                   'position': self.position,
                   'tag': self.location.category,
+                  'ajax_url': self.system.ajax_url,
                   }
 
         fragment.add_content(self.system.render_template('seq_module.html', params))