Add submission deadline to exercises and allow teachers to view their submissions

db/migrate/20200506093054_add_deadline_to_exercises.rb

@@ -0,0 +1,6 @@
+class AddDeadlineToExercises < ActiveRecord::Migration[5.2]
+  def change
+    add_column :exercises, :submission_deadline, :datetime, null: true, default: nil
+    add_column :exercises, :late_submission_deadline, :datetime, null: true, default: nil
+  end
+end