Fix for case described in https://github.com/syncloud/owncloud-setup/issues/19

Some apps might not define the same "nplural" value.
To avoid conflicts, the plural function is now generated per app.

Fixes #6427

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.

This reverts commit f12363d9, reversing
changes made to eba35d28.