Fix redirecting to login with a base url

It appears the base url wasn't being added.

Release-Notes: Fix redirecting to login with a base url
Change-Id: Ic1cfe2302a460f2effe098399496b45f2dd53740
(cherry picked from commit 9100b1bf22379e378ec78ffd2467de4d283dc5ed)
diff --git a/polygerrit-ui/app/elements/core/gr-router/gr-router.ts b/polygerrit-ui/app/elements/core/gr-router/gr-router.ts
index ad3e15c..30f4bf8 100644
--- a/polygerrit-ui/app/elements/core/gr-router/gr-router.ts
+++ b/polygerrit-ui/app/elements/core/gr-router/gr-router.ts
@@ -458,7 +458,9 @@
     // app.
     assign(
       window.location,
-      '/login/' + encodeURIComponent(returnUrl.substring(basePath.length))
+      `${basePath}/login/${encodeURIComponent(
+        returnUrl.substring(basePath.length)
+      )}`
     );
   }