ZP-581 #comment before processing a meeting request confirmation or cancellation, check if the current user is not the sender and only process it then. Should fix the auto-cancellation when cancelling one attendee.

git-svn-id: https://z-push.org/svn/z-push/trunk@1936 b7dd7b3b-3a3c-0410-9da9-bee62a6cc5b5