Enhancement: Show sharees as collapsed list of avatars

We've introduced a collapsed list of avatars of sharees in the `People` panel of the right sidebar.
On click we switch to showing the full list of sharees. With this additional intermediate state we
were able to clean up the UI a bit for easier cognitive load.

https://github.com/owncloud/web/pull/5758
https://github.com/owncloud/web/issues/5736
