[INTERNAL] retrieving nicknames now works again

But those static methods are still misplaced in the User class.

Change-Id: If5045d3639d86f56ba6fa0a441be6c31d77eaaf6
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1435
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>