[token-server + cashtab] Implement recaptcha for token rewards

Summary:
Recaptcha implementation on the claimxec endpoint has been successful enough to markedly increase the number of CACHET claims, particularly those going to address that was once frequently claiming and consolidating XEC rewards.

So, push recaptchas to the CACHET claim endpoint as well.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16868