Make appended_id for WMD editor a little less hacky by getting it from data-id of the 'reply-body' element instead of from _id of the wrapper.

lms/static/coffee/src/discussion/utils.coffee

@@ -143,8 +143,7 @@ class @DiscussionUtil
 
   @makeWmdEditor: ($content, $local, cls_identifier) ->
     elem = $local(".#{cls_identifier}")
-    id = $content.attr("_id")
-    #TODO: I think this is kind of a hack. At the least it should use data-id instead of _id
+    id = elem.data("id")
     appended_id = "-#{cls_identifier}-#{id}"
     imageUploadUrl = @urlFor('upload')
     _processor = (_this) ->

lms/templates/discussion/_underscore_templates.html

@@ -36,7 +36,7 @@
         <form class="local discussion-reply-new" data-id="${'<%- id %>'}">
             <h4>Post a response:</h4>
             <ul class="discussion-errors"></ul>
-            <div class="reply-body"></div>
+            <div class="reply-body" data-id="${'<%- id %>'}"></div>
             <div class="reply-post-control">
                 <a class="discussion-submit-post control-button" href="#">Submit</a>
             </div>

lms/templates/discussion/mustache/_inline_discussion.mustache

@@ -1,6 +1,6 @@
 <section class="discussion">
   {{#threads}}
-    <article class="discussion-thread" id="thread_{{id}}" _id="{{id}}">
+    <article class="discussion-thread" id="thread_{{id}}">
     </article>
   {{/threads}}
 </section>

lms/templates/discussion/mustache/_inline_thread.mustache

@@ -19,7 +19,7 @@
     <form class="local discussion-reply-new"  data-id="{{id}}">
         <h4>Post a response:</h4>
         <ul class="discussion-errors"></ul>
-        <div class="reply-body" _id="{{id}}"></div>
+        <div class="reply-body" data-id="{{id}}"></div>
         <div class="reply-post-control">
             <a class="discussion-submit-post control-button" href="#">Submit</a>
         </div>