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/reverseproxy.go

@@ -13,6 +13,7 @@ import (
 	"code.gitea.io/gitea/modules/log"
 	"code.gitea.io/gitea/modules/setting"
 	"code.gitea.io/gitea/modules/web/middleware"
+	"code.gitea.io/gitea/services/mailer"
 
 	gouuid "github.com/google/uuid"
 )
@@ -112,5 +113,7 @@ func (r *ReverseProxy) newUser(req *http.Request) *models.User {
 		return nil
 	}
 
+	mailer.SendRegisterNotifyMail(user)
+
 	return user
 }