Move to Stripe Prices

[dracos]

As well as being the modern Stripe way of doing it, this makes it easier to switch to different prices because we're not reliant on particular Stripe price (plan) IDs. A switch would then involve:

• Creating new Prices, including calls metadata entry
• Updating the price IDs in the settings and redeploying (for new subs)
• Emailing everyone to give them notice of the price change
• Running the script to schedule in everyone else's change after their next period (the end of the one after their current one)

Our Stripe test instance has new Prices, and the staging MapIt has example config in place with the right IDs, if you want to see how that's set up.