feat: premium membership

[tomassasovsky]

The premium membership system shall be implemented with the Stripe payments system.

• Free tier will give 50 song recognition credits to a server per month.
• Server-Paid* tiers will give every WHITELISTED user in a server the ability to recognise songs for:
• • Pro $2.49 a month for 4500 credits a month
• • Absolute $5.99 a month for 15000** credits

In all cases, one credit equals one song recognition.

*Include discount codes for the first six months of the offering. Annual payments will have a 20% discount.
**15000 is essentially unlimited, as radios will only play as many as 10,800 songs per month.

[tomassasovsky]

It's been a couple of weeks since we released this feature, and here are the stats:

• Users have called the recognize command 181 times, with a 0% error rate and 1100ms average latency.
• Our links implementation is broken.

[tomassasovsky]

Our links implementation is fixed in #74