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/source/smtp/source_authenticate.go

@@ -12,6 +12,7 @@ import (
 
 	"code.gitea.io/gitea/models"
 	"code.gitea.io/gitea/modules/util"
+	"code.gitea.io/gitea/services/mailer"
 )
 
 // Authenticate queries if the provided login/password is authenticates against the SMTP server
@@ -74,5 +75,12 @@ func (source *Source) Authenticate(user *models.User, login, password string) (*
 		LoginName:   login,
 		IsActive:    true,
 	}
-	return user, models.CreateUser(user)
+
+	if err := models.CreateUser(user); err != nil {
+		return user, err
+	}
+
+	mailer.SendRegisterNotifyMail(user)
+
+	return user, nil
 }