Add Nostr bunker login

Adds a SECRET Nostr bunker login proof-of-concept, only accessible by visiting /login/nostr directly.

Run nak bunker gleasonator.dev/relay then paste the bunker string to log in.

Later I would like to work this into the login flow, but for now we just need some people to be able to access it.

This only works if the bunker is configured to use the Ditto server's own relay. Ditto does not connect to other relays (yet) for signing.