Remove sidebar class from forum nav

This will enable much cleanup of the relevant SASS rules.

common/static/coffee/spec/discussion/view/discussion_thread_list_view_spec.coffee

@@ -147,7 +147,7 @@ describe "DiscussionThreadListView", ->
                 </div>
             </div>
         </script>
-        <div class="sidebar"></div>
+        <div class="forum-nav"></div>
         """
         @threads = [
           makeThreadWithProps({
@@ -176,7 +176,7 @@ describe "DiscussionThreadListView", ->
         spyOn($, "ajax")
 
         @discussion = new Discussion([])
-        @view = new DiscussionThreadListView({collection: @discussion, el: $(".sidebar")})
+        @view = new DiscussionThreadListView({collection: @discussion, el: $(".forum-nav")})
         @view.render()
 
     makeThreadWithProps = (props) ->
@@ -196,7 +196,7 @@ describe "DiscussionThreadListView", ->
 
     makeView = (discussion) ->
       return new DiscussionThreadListView(
-          el: $(".sidebar"),
+          el: $(".forum-nav"),
           collection: discussion
       )
 

common/static/coffee/src/discussion/discussion_router.coffee

@@ -8,7 +8,7 @@ if Backbone?
         @discussion = options['discussion']
         @course_settings = options['course_settings']
 
-        @nav = new DiscussionThreadListView(collection: @discussion, el: $(".sidebar"))
+        @nav = new DiscussionThreadListView(collection: @discussion, el: $(".forum-nav"))
         @nav.on "thread:selected", @navigateToThread
         @nav.on "thread:removed", @navigateToAllThreads
         @nav.on "threads:rendered", @setActiveThread

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

@@ -89,7 +89,7 @@ if Backbone?
       discussionsBodyTop = if discussionBody[0] then discussionBody.offset().top
       discussionsBodyBottom = discussionsBodyTop + discussionBody.outerHeight()
 
-      sidebar = $(".sidebar")
+      sidebar = $(".forum-nav")
       if scrollTop > discussionsBodyTop - @sidebar_padding
         sidebar.css('top', scrollTop - discussionsBodyTop + @sidebar_padding);
       else