Account validation by an administrator

[laurentj]

When registering, after the user validates his email, he can sign in. In some case, the administrator may want to validate the registration before the user sign in.

• implement new status flag
• The feature should be activated by configuration.
• (optional?) new comment field for the user to explain why he wants to register
• implement a zone for jauthdb_admin to show users awaiting after a validation
• modify the email template and the message after the registration, to tell to wait after an administrator validation
• send an email to the user after validating (or no) him