Private links: Retrieve link through propfind property #6020

Private links: Retrieve link through propfind property #6020

* The sharing ui does a propfind anyway: use that to query the new
property as well!
* For the socket api, asynchronously query the server for the right url
when an action that needs it is triggered.

The old, manually generated URL will be used as fallback in case the
server doesn't support the new property or the property can't be
retrieved for some reason.

Depends on owncloud/core#29021