feat: sitemap settings singleton with content types and additional addresses

- one settings tile opening the golem15.sitemap.settings singleton
- Files tab (site address, file size, S1 files partial), Additional
  addresses repeater, Content types tab built from signpost at boot
- golem15_sitemap_types; a type switched off leaves the files (reason type)
- a save validates (errors.*) and dispatches a rebuild after commit

controllers/admin_registry.go

@@ -11,5 +11,6 @@ import (
 func AdminControllers(app func() *backpack.App) []pact.AdminController {
 	return []pact.AdminController{
 		definitionsAdminController{app: app},
+		settingsAdminController{app: app},
 	}
 }