Load avatars via the new standard API

At this moment we load avatars via a private API/URL. https://github.com/nextcloud/server/pull/4062 introduces a public webdav API to load avatars including some meta information. SO I'd say we should move to this API as soon as it becomes available while we still need to keep the "old way" for a while to support older Server versions of Nextcloud and also support avatars a little longer for oC which doesn't have this API (I Guess).