Add segment.io tracking for legacy instructor dashboard

LMS-1296

lms/static/coffee/src/instructor_dashboard_tracking.coffee

 if $('.instructor-dashboard-wrapper').length == 1
-    analytics.track "Loaded an Instructor Dashboard Page", 
+    analytics.track "Loaded a Legacy Instructor Dashboard Page", 
         location: window.location.pathname
         dashboard_page: $('.navbar .selectedmode').text()

lms/templates/courseware/instructor_dashboard.html

@@ -131,7 +131,7 @@ function goto( mode)
     <h1>${_("Instructor Dashboard")}</h1>
 
     <div class="wrapper-msg urgency-low warning">
-      <div class="msg""
+      <div class="msg"
         <p>${_("You are using the legacy instructor dashboard, which will be going away in the near future. Please use the Standard Dashboard. If it is missing functionality, please let us know.")}
         <a href="${ standard_dashboard_url }">${_("Return to the Standard Dashboard")}  <i class="icon-angle-right"></i></a></p>
       </div>