Update devise to v3.2.4.
Add the generated secret key.
Pass the correct token in to the new user account mailer.

As new users are sent a password reset link, rather than
creating an account for themselves, we don't use the standard
reset password template. Therefore we need to replicate Devise's
new way of generating a token.
Refactor UserMailer spec to use let blocks.
Remove copy regarding the "new" application.
Merge pull request #222 from alphagov/update-devise

Update Devise to v3.2.4