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

app/models/course.rb

@@ -1,8 +1,9 @@
 class Course < ApplicationRecord
   has_many :lectures, dependent: :destroy
+  has_many :work_items, dependent: :destroy
 
-  # A course has a standalone connection to its recordings. To be shown they must be associated with a lecture but we
-  # track those not associated with a lecture to avoid duplication.
+  # A course has a standalone connection to its recordings. To be displayed they must be associated with a lecture but
+  # we them independently to avoid re-importing lectures.
   has_many :recordings, dependent: :destroy
 
   def import_from_calendar!

app/models/lecture.rb

@@ -2,7 +2,7 @@ class Lecture < ApplicationRecord
   belongs_to :course
   has_one :recording, dependent: :nullify
 
-  has_many :tracked_time_entries, dependent: :destroy
+  has_many :tracked_time_entries, dependent: :destroy, as: :subject
 
   enum :kind, [
     :lecture,

app/models/tracked_time_entry.rb

@@ -1,5 +1,5 @@
 class TrackedTimeEntry < ApplicationRecord
-  belongs_to :lecture
+  belongs_to :subject, polymorphic: true
 
   enum :kind, [
     :concurrent,

app/models/work_item.rb

@@ -1,3 +1,4 @@
 class WorkItem < ApplicationRecord
   belongs_to :course
+  has_many :tracked_time_entries, dependent: :destroy, as: :subject
 end