Promo code landing route

[adamkdean]

We need promo codes on sign up to be accessible via an initial landing route, e.g. /promo/WELCOME10 etc. So for example you'd hit account.edge.network/promo/<code> and that sets a promo code in a store, and then it redirects to create an account, but keeps the promo code in memory and applies it once account is created.

Also important to show that the code is active during the create account process.