Fix login redirect problems. (#270)

After login, my browser gets a 404 for `/wireguard`. `wireguard` might
not be explicitly set by `BASE_PATH`, so just use the `{{.basePath}}`
instead.

Fixes #259.

templates/login.html

@@ -85,7 +85,7 @@
         if (nextURL) {
             window.location.href = nextURL;
         } else {
-            window.location.href = '/wireguard/';
+            window.location.href = '/{{.basePath}}';
         } 
     }
 </script>