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

db/migrate/20231007144716_make_tracked_time_entry_polymorphic.rb

@@ -0,0 +1,19 @@
+class MakeTrackedTimeEntryPolymorphic < ActiveRecord::Migration[7.1]
+  def change
+    add_reference :tracked_time_entries, :subject, polymorphic: true
+
+    reversible do |dir|
+      dir.up do
+        TrackedTimeEntry.all.each do |entry|
+          entry.update!(subject: Lecture.find(entry.lecture_id))
+        end
+      end
+
+      dir.down do
+        TrackedTimeEntry.all.each do |entry|
+          entry.update!(lecture_id: entry.subject_id)
+        end
+      end
+    end
+  end
+end

db/migrate/20231007145018_remove_lecture_association.rb

@@ -0,0 +1,7 @@
+class RemoveLectureAssociation < ActiveRecord::Migration[7.1]
+  def change
+    remove_reference :tracked_time_entries, :lecture, null: false, foreign_key: true
+    change_column_null :tracked_time_entries, :subject_id, false
+    change_column_null :tracked_time_entries, :subject_type, false
+  end
+end