Enhancement: Lazy file avatar loading

We've changed the way how large file lists get rendered.
In some cases where we had a long list of files, the loading of avatars could lead to long waiting times till the first paint happens.

Now we first render the list of files, load the associated avatars in the background and then update the ui.

https://github.com/owncloud/web/pull/5073
https://github.com/owncloud/web/issues/4973
