Show cohorts on the new instructor dashboard

TNL-161

Remove placeholder text

lms/static/js/models/cohort.js

@@ -0,0 +1,11 @@
+(function(Backbone) {
+    var CohortModel = Backbone.Model.extend({
+        idAttribute: 'id',
+        defaults: {
+            name: '',
+            user_count: 0
+        }
+    });
+
+    this.CohortModel = CohortModel;
+}).call(this, Backbone);