Switch to use openid as identifer rather than the nickname

- This allows better support of other OpenID providers than FAS
- Usernames are only used to build the URLs and check the admins
- Save the email and the openid url in the database
Simplifies the openid URLs and catch exception if we two emails identical

The two emails problem could raise if someone has 2 openid providers providing
authentification for the same email (ie: FAS and it's own openid provider?)