Drop "unrolled/render" package (#23965)

None of the features of `unrolled/render` package is used. 

The Golang builtin "html/template" just works well. Then we can improve
our HTML render to resolve the "$.root.locale.Tr" problem as much as
possible.

Next step: we can have a template render pool (by Clone), then we can
inject global functions with dynamic context to every `Execute` calls.
Then we can use `{{Locale.Tr ....}}` directly in all templates , no need
to pass the `$.root.locale` again and again.

services/auth/sspi_windows.go

@@ -23,7 +23,6 @@ import (
 
 	gouuid "github.com/google/uuid"
 	"github.com/quasoft/websspi"
-	"github.com/unrolled/render"
 )
 
 const (
@@ -48,7 +47,7 @@ var (
 // On successful authentication returns a valid user object.
 // Returns nil if authentication fails.
 type SSPI struct {
-	rnd *render.Render
+	rnd *templates.HTMLRender
 }
 
 // Init creates a new global websspi.Authenticator object