Fix webroot for update page

On the update page, config.js was missing which caused oc_webroot to not
be available. That would trigger the faulty oc_webroot fallback that
didn't take URLs like "/owncloud/index.php/files/apps" into account.

This fix adds config.js in the update page and also a fix for the
oc_webroot fallback, in case it is used elsewhere.
Merge pull request #6390 from owncloud/migration-fixwebroot

Fix webroot for update page