Directly call toggleArrow in sequence render

There might be a chance that an error got thrown after 'contentChanged'
event was fired and stop the `toggleArrow()` method to be called. This
will make sure that the navigation will still be working even some
element in the content is failed.

lms/static/coffee/spec/modules/sequence_spec.coffee

@@ -24,7 +24,6 @@ describe 'Sequence', ->
       expect(titles).toEqual ['Video 1', 'Video 2', 'Sample Problem']
 
     it 'bind the page events', ->
-      expect(@sequence.element).toHandleWith 'contentChanged', @sequence.toggleArrows
       expect($('#sequence-list a')).toHandleWith 'click', @sequence.goto
 
     it 'render the active sequence content', ->
@@ -76,6 +75,7 @@ describe 'Sequence', ->
       spyOn $, 'postWithPrefix'
       @sequence = new Sequence '1', @items, 'sequence'
       spyOnEvent @sequence.element, 'contentChanged'
+      spyOn(@sequence, 'toggleArrows').andCallThrough()
 
     describe 'with a different position than the current one', ->
       beforeEach ->
@@ -105,6 +105,9 @@ describe 'Sequence', ->
       it 'update the position', ->
         expect(@sequence.position).toEqual 1
 
+      it 're-update the arrows', ->
+        expect(@sequence.toggleArrows).toHaveBeenCalled()
+
       it 'trigger contentChanged event', ->
         expect('contentChanged').toHaveBeenTriggeredOn @sequence.element
 

lms/static/coffee/src/modules/sequence.coffee

@@ -9,7 +9,6 @@ class @Sequence
     $(selector, @element)
 
   bind: ->
-    @element.bind 'contentChanged', @toggleArrows
     @$('#sequence-list a').click @goto
 
   buildNavigation: ->
@@ -43,6 +42,7 @@ class @Sequence
 
       MathJax.Hub.Queue(["Typeset", MathJax.Hub])
       @position = new_position
+      @toggleArrows()
       @element.trigger 'contentChanged'
 
   goto: (event) =>