Registration in Antarcticle using Poulpe

When the plugin is enabled and user tries to register:

• Instead of saving the user into Antarcticle internal database, user's data should be sent via REST to Poulpe.
• If Poulpe returns success, then user registration continues as it was before (note, that Poulpe works with the same DB now, thus user should be there by that time).
• If Poulpe answers with validation error, user should be shown with respective error message
• If Poulpe answers with generic error, user should be shown with "Some unexpected error occurred, please contact administrator or try later"
• If Poulpe is not accessible, the error is: "Authentication service is not available right now. Please, contact administrator or try later"