Move more forum nav rendering logic to front end

The cohort selector is rendered only for a privileged user in a cohorted
course, and that logic is now in an Underscore template instead of Mako.

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

@@ -8,7 +8,11 @@ if Backbone?
         @discussion = options['discussion']
         @course_settings = options['course_settings']
 
-        @nav = new DiscussionThreadListView(collection: @discussion, el: $(".forum-nav"))
+        @nav = new DiscussionThreadListView(
+            collection: @discussion,
+            el: $(".forum-nav"),
+            courseSettings: @course_settings
+        )
         @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

@@ -13,7 +13,8 @@ if Backbone?
       "change .forum-nav-filter-main-control": "chooseFilter"
       "change .forum-nav-filter-cohort-control": "chooseCohort"
 
-    initialize: ->
+    initialize: (options) ->
+      @courseSettings = options.courseSettings
       @displayedCollection = new Discussion(@collection.models, pages: @collection.pages)
       @collection.on "change", @reloadDisplayedCollection
       @discussionIds=""
@@ -121,7 +122,12 @@ if Backbone?
 
     render: ->
       @timer = 0
-      @$el.html(@template())
+      @$el.html(
+        @template({
+          isCohorted: @courseSettings.get("is_cohorted"),
+          isPrivilegedUser: DiscussionUtil.isPrivilegedUser()
+        })
+      )
       @$(".forum-nav-sort-control").val(@collection.sort_preference)
 
       $(window).bind "load", @updateSidebar