Fix mail subjects for wallposts

CLUB-758 #resolve

Subject was wrong because full_name isn't a property but a method on Member model. 
Subject: <bound method Member.full_name of <Member: stichtinglasonrisa@outlook.com>> heeft gereageerd op jouw post.
Booking doesn't have a short_name and will therefore display full name I think.
Let's ask what's preferable. I'd go for short_name if available.
But I agree that full_name should be a property (get_full_name is the method) but I was a bit hesitant to change that without testing thoroughly.