Add index for access_token (#21908)

The `token_last_eight` field has been used in `GetAccessTokenBySHA `:

models/migrations/v1_19/v235.go

@@ -0,0 +1,17 @@
+// Copyright 2022 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package v1_19 //nolint
+
+import (
+	"xorm.io/xorm"
+)
+
+func AddIndexForAccessToken(x *xorm.Engine) error {
+	type AccessToken struct {
+		TokenLastEight string `xorm:"INDEX token_last_eight"`
+	}
+
+	return x.Sync(new(AccessToken))
+}