Add Index to action.user_id (#27403)

Another Column that needs a Index. Found at
https://codeberg.org/forgejo/discussions/issues/61#issuecomment-1258744.

Co-authored-by: Giteabot <teabot@gitea.io>

models/migrations/v1_21/v279.go

@@ -0,0 +1,16 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package v1_21 //nolint
+
+import (
+	"xorm.io/xorm"
+)
+
+func AddIndexToActionUserID(x *xorm.Engine) error {
+	type Action struct {
+		UserID int64 `xorm:"INDEX"`
+	}
+
+	return x.Sync(new(Action))
+}