[GUI] show the nickname of the inviter on the invitation page

I was just browsing through A. Waldmanns DEFEKT Katalog and stumbled
across this "excellent" design. Changed the logic, only the project
information is now transmitted so that the invitee side can properly
generate additional string content for the invitation description.

Change-Id: I677bfa525373c58cecf672c686d01f838ab7e940
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1207
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>