Incognito mode

Allow to set an incognito mode to hide the user even if he is logged in.
This is for example useful if we access files via public.php. Where we always want to act like no user is logged in, even if you are logged-in in parallel. Will be used here https://github.com/owncloud/core/pull/5976 and probably also in other places.

cc @PVince81 @karlitschek This is implemented like discussed this morning.


Maybe some other people with some knowledge regarding user management can also have a look... @blizzz @DeepDiver1975 @bartv2 maybe? Thanks!