Trust Google and Yahoo to return valid email addresses

Both Google and Yahoo guarantee that the email address they return
during the OpenID authentication are emails that they have already
verified:

  http://stackoverflow.com/q/5639419

Therefore special case these OpenID providers and automatically
activate the new users account without requiring a separate email
verification step.

This therefore reduces the signup procedure by one step and makes
it easier for new users of these OpenID providers, which cover the
majority of users.