Buggy delete links in overview

The delete links in the invitation overview didn't allow expired invitations to be deleted. I fixed that (see attached patch). I took the liberty to change the link text "delete" into "delete invitation" to allow for more flexible translation (avoiding conflicts with other "delete" strings). I also took the liberty to remove the "delete" text al together for invitations with status Joined as it's quite confusing to show this text when actually you can not delete.

The cvs diff left me with a patch containing carriage returns which I removed from the patch.