add checkbox at registration "terms&conditions & privacy"

[Marc-pi]

• add optional field in module config to set a "Validate Website Policy" checkbox at registration (legal obligation in Europe)
• legal term&condition/privacy document must be set with versioning in module admin
  • manage versioning and make it possible for user to print/down the pdf version of it
  • name, version (either use Page module but we need pdf), or special feature with User module to manage versioning and content
• this field validation must be saved in DB with validation date, per document version and per user
  • once validated, a user cannot unvalidate (he can only validate a new version of the policy
• once a new version is released, the user will be asked to validate the new version (for instance when making a new order)