Bugfix: Add index route for the OC10 integration

Added an index route for the OC10 integration which gets called when opening http://your-server/index.php/apps/web.
The route basically redirects to the same URL while appending /index.html, as this is the correct URL for
accessing the Web UI. Setting Web as default layout would result in an endless redirect loop otherwise.

https://github.com/owncloud/web/pull/5201
https://github.com/owncloud/core/issues/38799
