Order referral code

https://www.pivotaltracker.com/story/show/56679882

This is a pre-requisite for the Permaculture harvest benchmarking project we're starting on next week.  I promised Permaculture Melbourne a discount if they could get people to sign up for Growstuff paid accounts, so we have to track that somehow.  This needs to be in place on the production website by the start date (next Tuesday), hence the rush/not pairing/etc, as nobody's available to pair at present.  However, I'd like to walk through it with whoever code reviews it, before merging.