Allow white labeling loading logo (#36174)

* Allow to whitelabeling loading logo

* Add loading_logo to documentation

* Change loading_logo to loading_logo_url

pkg/api/dtos/index.go

@@ -26,6 +26,7 @@ type IndexViewData struct {
 	AppTitle                string
 	Sentry                  *setting.Sentry
 	ContentDeliveryURL      string
+	LoadingLogo             template.URL
 	// Nonce is a cryptographic identifier for use with Content Security Policy.
 	Nonce string
 }