Internationalize the forums post Markdown editor

This includes removing the "PREVIEW" label for the preview pane from CSS
and adding it to the generated DOM.

common/static/coffee/src/discussion/views/thread_response_view.coffee

@@ -35,7 +35,7 @@ if Backbone?
 
     hideEditorChrome: ->
       @$('.wmd-button-row').hide()
-      @$('.wmd-preview').hide()
+      @$('.wmd-preview-container').hide()
       @$('.wmd-input').css({
         height: '35px',
         padding: '5px'
@@ -44,7 +44,7 @@ if Backbone?
 
     showEditorChrome: ->
       @$('.wmd-button-row').show()
-      @$('.wmd-preview').show()
+      @$('.wmd-preview-container').show()
       @$('.comment-post-control').show()
       @$('.wmd-input').css({
         height: '125px',