Add reminder datetime and done-flag

db/migrate/20170917093221_add_reminder_to_activities.rb

@@ -0,0 +1,6 @@
+class AddReminderToActivities < ActiveRecord::Migration[5.0]
+  def change
+    add_column :activities, :reminder_at, :datetime
+    add_column :activities, :reminder_done, :boolean
+  end
+end