Incorporate strfry policies

Fixes #15

I've published the strfry-policies package to npm. This MR incorporates them into the Minds relay in a way that's pluggable and configurable. Eventually it makes sense to rewrite the Redis rate limiter as a policy and fit it into this system.

I'm also making the assumption we'll remove the pubkey whitelist from Minds backend and control that here instead. For that to be doable, the /api/v3/nostr/event endpoint should require an API token which we'll configure on the relay.

The endgoal of this is to build a good enough system that prevents spam, so the floodgates can be opened.