Fix an `ActionView::Template::Error: undefined method 'verification_key'
for nil` error in the welcome_user mailer when a user signs up without
an email address.
Caused by the fact that we now render mail templates regardless of
whether the user has an email address, and then skip sending the email
only after the mail template is rendered.
Fix mailers to not attempt deliveries to invalid or nonexistent email
addresses. This usually happened when someone changed their email, and
we tried to send a confirmation email to a nonexistent address.
