Use preferred username as the omniauth nickname

There's a difference of opinion on what nickname means between the
OmniAuth Auth Hash Schema and the OpenID Connect spec.

According to the OpenID Connect spec
> Casual name of the End-User that may or may not be the same as the
> given_name. For instance, a nickname value of Mike might be returned
> alongside a given_name value of Michael.

According to OmniAuth:
> The username of an authenticating user (such as your @-name from
> Twitter or GitHub account name)