Add WorkItem model for Problem sheets and equivalent

db/migrate/20231007144031_create_work_items.rb

@@ -0,0 +1,11 @@
+class CreateWorkItems < ActiveRecord::Migration[7.1]
+  def change
+    create_table :work_items do |t|
+      t.string :title, null: false
+      t.references :course, null: false, foreign_key: true
+      t.datetime :due_date, null: false
+
+      t.timestamps
+    end
+  end
+end