Fix rider creation
Fix rider creation

Having made the donation_page a property and introducing fundraising into the rider serializer, the process of creating a new rider has become broken as it expects these fields to be writeable. This fixes this.

Signed-off-by: Michael Willmott <17b9e1c64588c7fa6419b4d29dc1f4426279ba01@knodium.com>

mwillmott/techbikers,Techbikers/techbikers,mwillmott/techbikers,Techbikers/techbikers,mwillmott/techbikers,Techbikers/techbikers,mwillmott/techbikers,Techbikers/techbikers