Clean template/helper.go (#23922)

The first step of #23328

This PR cleans:

1. Dead function like `SubStr`
2. Unnecessary function like `UseHTTPS`, it should be provided by
`window.origin`
3. Duplicate function like `IsShowFullName`, there was already a
`DeafultShowFullName`

I have searched these removed functions globally, no use in code.

routers/web/admin/config.go

@@ -115,6 +115,7 @@ func Config(ctx *context.Context) {
 
 	ctx.Data["CustomConf"] = setting.CustomConf
 	ctx.Data["AppUrl"] = setting.AppURL
+	ctx.Data["AppBuiltWith"] = setting.AppBuiltWith
 	ctx.Data["Domain"] = setting.Domain
 	ctx.Data["OfflineMode"] = setting.OfflineMode
 	ctx.Data["DisableRouterLog"] = setting.Log.DisableRouterLog