Add scheduled email feature for reminders

spec/mailers/previews/reminder_mailer_preview.rb

@@ -0,0 +1,7 @@
+# Preview all emails at http://localhost:3000/rails/mailers/reminder_mailer
+class ReminderMailerPreview < ActionMailer::Preview
+  # Preview this email at http://localhost:3000/rails/mailers/reminder_mailer/schedule_email
+  def schedule_email
+    ReminderMailer.schedule_email(FactoryBot.build(:reminder), 'to@example.com')
+  end
+end