Enhancement: Webfinger redirect app

We've added an app with the name `webfinger` which queries the oCIS webfinger service for the url of the oCIS instance of the user and performs a redirect. This app is not meant to be used in a standard deployment in its current state. It only uses web and its extension system as a platform. In the future this will become a multi tenancy select.

https://github.com/owncloud/web/issues/8883
https://github.com/owncloud/web/pull/8884
https://github.com/owncloud/web/pull/8950
https://github.com/owncloud/web/pull/8952
