Send registration email on user autoregistration (#16523)

When users login and are autoregistered send email notification.

Fix #16178

* Protect public functions within the mailer by testing if the mailer is configured

Signed-off-by: Andrew Thornton <art27@cantab.net>

Co-authored-by: 6543 <6543@obermui.de>

services/auth/sspi_windows.go

@@ -16,6 +16,7 @@ import (
 	"code.gitea.io/gitea/modules/templates"
 	"code.gitea.io/gitea/modules/web/middleware"
 	"code.gitea.io/gitea/services/auth/source/sspi"
+	"code.gitea.io/gitea/services/mailer"
 
 	gouuid "github.com/google/uuid"
 	"github.com/quasoft/websspi"
@@ -197,6 +198,9 @@ func (s *SSPI) newUser(username string, cfg *sspi.Source) (*models.User, error)
 	if err := models.CreateUser(user); err != nil {
 		return nil, err
 	}
+
+	mailer.SendRegisterNotifyMail(user)
+
 	return user, nil
 }