Add email confirmation field to registration form

What does this MR do?

Fixes #3053 (closed)

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 validates_confirmation_of is the same thing Devise uses for password. The trick is that it only requires the confirmation if the confirmation parameter is also passed. We do this on the registration form so it validates that the emails match. However, it also means it will not negatively affect any other place an email can be changed - API or profile settings.