referrals: `referred_by` column

[Evanfeenstra]

Context

referral program for hunters

Design

This is a frontend and backend task.

In the frontend, if you visit the page and a url query exists with value of p (like ?p=123456), then save it as referred_by in localStorage ONLY if a referred_by value already doesn't exist in localStorage.

On signup, if referred_by UUID exists in localStorage, then send it to the backend, find that person's ID, and store it on the newly created Person record as a new SQL column called referred_by