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.
What if a user doesn't have a first name? Booking uses the full_name reference in their emails. Maybe we should add the property decorator to full_name property?