[Fixes #35791223] Clear the WMD preview after posting a new post.

lms/static/coffee/src/discussion/views/new_post_view.coffee

@@ -147,6 +147,7 @@ if Backbone?
                   @$(".new-post-body textarea").val("").attr("prev-text", "")
                   @$(".new-post-tags").val("")
                   @$(".new-post-tags").importTags("")
+                  @$(".wmd-preview p").html("")
                   @collection.add thread
 
       setActiveItem: (event) ->