Allow options to disable user deletion from the interface on app.ini (#29275)

Extract from #20549

This PR added a new option on app.ini `[admin]USER_DISABLED_FEATURES` to
allow the site administrator to disable users visiting deletion user
interface or allow.
This options are also potentially allowed to define more features in
future PRs.

---------

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

modules/setting/admin.go

@@ -3,14 +3,22 @@
 
 package setting
 
+import "code.gitea.io/gitea/modules/container"
+
 // Admin settings
 var Admin struct {
 	DisableRegularOrgCreation bool
 	DefaultEmailNotification  string
+	UserDisabledFeatures      container.Set[string]
 }
 
 func loadAdminFrom(rootCfg ConfigProvider) {
-	mustMapSetting(rootCfg, "admin", &Admin)
 	sec := rootCfg.Section("admin")
+	Admin.DisableRegularOrgCreation = sec.Key("DISABLE_REGULAR_ORG_CREATION").MustBool(false)
 	Admin.DefaultEmailNotification = sec.Key("DEFAULT_EMAIL_NOTIFICATIONS").MustString("enabled")
+	Admin.UserDisabledFeatures = container.SetOf(sec.Key("USER_DISABLED_FEATURES").Strings(",")...)
 }
+
+const (
+	UserFeatureDeletion = "deletion"
+)