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/pam/source_authenticate.go

@@ -11,6 +11,7 @@ import (
 	"code.gitea.io/gitea/models"
 	"code.gitea.io/gitea/modules/auth/pam"
 	"code.gitea.io/gitea/modules/setting"
+	"code.gitea.io/gitea/services/mailer"
 
 	"github.com/google/uuid"
 )
@@ -58,5 +59,12 @@ func (source *Source) Authenticate(user *models.User, login, password string) (*
 		LoginName:   login, // This is what the user typed in
 		IsActive:    true,
 	}
-	return user, models.CreateUser(user)
+
+	if err := models.CreateUser(user); err != nil {
+		return user, err
+	}
+
+	mailer.SendRegisterNotifyMail(user)
+
+	return user, nil
 }