Added WorkItems as a new subject for TrackedTimeEntries.

- Made TrackedTimeEntries polymorphic to belong to both Lecture and WorkItems models
- Added associated migrations for existing data
- Added to administrate UI

config/routes.rb

@@ -2,6 +2,7 @@ Rails.application.routes.draw do
   namespace :admin do
     resources :courses
     resources :lectures
+    resources :work_items
     resources :tracked_time_entries
     resources :recordings