Improvements to one-step registration

- Use an Enumeration instead of String for user registration state. Value can be one of
    * NotValidated (only in case of one-step registration, user with pending email validation)
    * ValidIdentity (only for external providers, when email address not available)
    * ValidEmail (for both UsernamePassword and external providers, when email address is available and validated)
- Handle properly the new state field for external providers. State is set to either ValidIdentity or ValidEmail, depending if email information is available from external provider.
- Do not filter NotValidated user in SecuredActionBuilder, to allow granting access to NotValidated users on some pages
- Add configuration option to define the minimum user state required to access secured pages. Defaults to ValidIdentity to keep existing behavior (do not allow NotValidated users, but allow users logged in with external provider without email information)
- Port feature to Java API
- Fix behavior of signUpVerification Action to take into account the configuration options for signupSkipLogin and onSignUpGoTo
- Allow customization of message in notAuthorized page through request.flash to provide feedback on required access level