Fix webpack chunk loading with STATIC_URL_PREFIX (#11526)

Previously, we had only set __webpack_public_path__ to a path which
caused webpack chunks to be loaded from the current origin which is
incorrect when STATIC_URL_PREFIX points to another origin.

This should fix the issue curretnly seen on gitea.com.

Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: Lauris BH <lauris@nix.lv>

web_src/js/publicpath.js

@@ -1,5 +1,10 @@
-// This sets up webpack's chunk loading to load resources from the 'public'
-// directory. This file must be imported before any lazy-loading is being attempted.
+// This sets up the URL prefix used in webpack's chunk loading.
+// This file must be imported before any lazy-loading is being attempted.
+const {StaticUrlPrefix} = window.config;
 
+if (StaticUrlPrefix) {
+  __webpack_public_path__ = StaticUrlPrefix.endsWith('/') ? StaticUrlPrefix : `${StaticUrlPrefix}/`;
+} else {
   const url = new URL(document.currentScript.src);
   __webpack_public_path__ = url.pathname.replace(/\/[^/]*?\/[^/]*?$/, '/');
+}