Rename StaticUrlPrefix to AssetUrlPrefix (#15779)

Use a new name for this template/frontend variable to make it distinct
from the server variable StaticURLPrefix.

web_src/js/features/serviceworker.js

@@ -31,7 +31,7 @@ export default async function initServiceWorker() {
 
   if (UseServiceWorker) {
     try {
-      // normally we'd serve the service worker as a static asset from StaticUrlPrefix but
+      // normally we'd serve the service worker as a static asset from AssetUrlPrefix but
       // the spec strictly requires it to be same-origin so it has to be AppSubUrl to work
       await Promise.all([
         checkCacheValidity(),