Add email and password confirmation fields to registration form

It's too easy to mistype an email or password when signing up.
The support team is receiving an increasing number of requests
because users mistype their email. We can eliminate this problem
by requiring users to confirm the email before registering. The
same issue can occur for the password field so we should add
this, too. We should note that password confirmation is part
of the default Devise forms. I don't know why/when GitLab
removed it.