Refactor system setting (#27000) (#27452)

Backport #27000 by @wxiaoguang

This PR reduces the complexity of the system setting system.

It only needs one line to introduce a new option, and the option can be
used anywhere out-of-box.

It is still high-performant (and more performant) because the config
values are cached in the config system.


![image](f8cdd743-1145-41ab-9f8f-3996aa97d440)

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

models/migrations/v1_18/v227.go

@@ -4,10 +4,6 @@
 package v1_18 //nolint
 
 import (
-	"fmt"
-	"strconv"
-
-	"code.gitea.io/gitea/modules/setting"
 	"code.gitea.io/gitea/modules/timeutil"
 
 	"xorm.io/xorm"
@@ -22,42 +18,6 @@ type SystemSetting struct {
 	Updated      timeutil.TimeStamp `xorm:"updated"`
 }
 
-func insertSettingsIfNotExist(x *xorm.Engine, sysSettings []*SystemSetting) error {
-	sess := x.NewSession()
-	defer sess.Close()
-	if err := sess.Begin(); err != nil {
-		return err
-	}
-	for _, setting := range sysSettings {
-		exist, err := sess.Table("system_setting").Where("setting_key=?", setting.SettingKey).Exist()
-		if err != nil {
-			return err
-		}
-		if !exist {
-			if _, err := sess.Insert(setting); err != nil {
-				return err
-			}
-		}
-	}
-	return sess.Commit()
-}
-
 func CreateSystemSettingsTable(x *xorm.Engine) error {
-	if err := x.Sync(new(SystemSetting)); err != nil {
-		return fmt.Errorf("sync2: %w", err)
-	}
-
-	// migrate xx to database
-	sysSettings := []*SystemSetting{
-		{
-			SettingKey:   "picture.disable_gravatar",
-			SettingValue: strconv.FormatBool(setting.DisableGravatar),
-		},
-		{
-			SettingKey:   "picture.enable_federated_avatar",
-			SettingValue: strconv.FormatBool(setting.EnableFederatedAvatar),
-		},
-	}
-
-	return insertSettingsIfNotExist(x, sysSettings)
+	return x.Sync(new(SystemSetting))
 }