ServeFromSubPath: Redirect to URL with subpath when subpath missing (#66724)

* ServeFromSubPath: Redirect to URL with subpath when subpath missing

* Review fixes

* Added tests

* Use constant

* change to useMiddleware

* Update pkg/middleware/subpath_redirect.go

---------

Co-authored-by: Carl Bergquist <carl.bergquist@gmail.com>

pkg/api/http_server.go

@@ -602,6 +602,7 @@ func (hs *HTTPServer) addMiddlewaresAndStaticRoutes() {
 
 	if hs.Cfg.ServeFromSubPath && hs.Cfg.AppSubURL != "" {
 		m.SetURLPrefix(hs.Cfg.AppSubURL)
+		m.UseMiddleware(middleware.SubPathRedirect(hs.Cfg))
 	}
 
 	m.UseMiddleware(web.Renderer(filepath.Join(hs.Cfg.StaticRootPath, "views"), "[[", "]]"))
@@ -616,6 +617,7 @@ func (hs *HTTPServer) addMiddlewaresAndStaticRoutes() {
 
 	m.UseMiddleware(hs.ContextHandler.Middleware)
 	m.Use(middleware.OrgRedirect(hs.Cfg, hs.userService))
+
 	if !hs.Features.IsEnabled(featuremgmt.FlagAuthnService) {
 		m.Use(accesscontrol.LoadPermissionsMiddleware(hs.accesscontrolService))
 	}