[cashtab-faucet] Tighten IP rate limits

Summary: Tighten IP rate limits. We had expanded rate limits before as normal users would often get unacceptable recaptcha scores and have to retry. But recaptcha success is now binary.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18954