(red) Write failing test => SignUpsController#update sets
email_sent_on when email sent
Add email_sent datetime column and indexes to sign_ups table
(green) Make test pass => SignUpsController#update sets email_sent_on
when email sent
(red) Write failing test => SignUpsController#update does not send an
email if email_sent_on exists
(green) Make test passSignUpsController#update does not send an email
if email_sent_on exists
Refactor set_password_email to validate without errors on
http://validator.w3.org/check
Set set_password email to be Sidekiq delayed job