[token-server] Reject queries with insufficient recaptcha score

Summary:
Now that we have the data from yesterday, can see that most "success" recaptcha queries had a low score.

Implement a threshold. In local testing, sometimes I get as low as 0.7, so use this as a threshold. Most queries from suspected bots lower than this.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16858