Load avatars via the new standard API

[AndyScherzinger]

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).

[tobiasKaminsky]

Additionally to this all avatar loading must be non blocking. I had sometimes large delays when a server instance was not accessible. This should be done in the library, or?

[AndyScherzinger]

The loading should be done via the library yes, while the non blocking part might be done within the app or we must implement it in a way that all this is covered within lib code with weak references etc etc.

[tobiasKaminsky]

We can now also rely for >=NC13 that we always have an avatar: https://github.com/nextcloud/server/pull/6876

[AndyScherzinger]

Nice! 🎉