add firstname and lastname as default fields

[Marc-pi]

when using Complete Profile feature, we should not need to hack. those fields must be standard
username could be renamed to nickname in db and app code, to avoid confusion

[Marc-pi]

we have to clarify :

• username : hidden names to log in (used only if you not use email as login)
• display name / nickname
• real name : firsname and lastname

user must be able to set his displayname/nickname displayed on the website