Placeholder for wmd editors; specifically comment form

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

@@ -155,6 +155,7 @@ class @DiscussionUtil
 
   @makeWmdEditor: ($content, $local, cls_identifier) ->
     elem = $local(".#{cls_identifier}")
+    placeholder = elem.data('placeholder')
     id = elem.data("id")
     appended_id = "-#{cls_identifier}-#{id}"
     imageUploadUrl = @urlFor('upload')
@@ -162,6 +163,8 @@ class @DiscussionUtil
       (text) -> _this.postMathJaxProcessor(text)
     editor = Markdown.makeWmdEditor elem, appended_id, imageUploadUrl, _processor(@)
     @wmdEditors["#{cls_identifier}-#{id}"] = editor
+    if placeholder?
+      elem.find("#wmd-input#{appended_id}").attr('placeholder', placeholder)
     editor
 
   @getWmdEditor: ($content, $local, cls_identifier) ->

lms/templates/discussion/_underscore_templates.html

@@ -79,7 +79,8 @@
         <li class="new-comment response-local">
             <form class="comment-form" data-id="${'<%- wmdId %>'}">
                 <ul class="discussion-errors"></ul>
-                <div class="comment-body" data-id="${'<%- wmdId %>'}"></div>
+                <div class="comment-body" data-id="${'<%- wmdId %>'}"
+                data-placeholder="Add a comment..."></div>
                 <div class="comment-post-control">
                     <a class="discussion-submit-comment control-button" href="#">Submit</a>
                 </div>