[Cashtab] Add a blacklist to remove scams from agora

Summary:
Impressively, the first scam offers appeared (fake BUX) only minutes after agora launched in Cashtab.

Implement a simple blacklist so that we do not render such offers.

Going forward will develop a more robust solution that does not require a diff for every addition. But this is a start.

Test Plan: npm test

Reviewers: #bitcoin_abc, Kronkmeister

Reviewed By: #bitcoin_abc, Kronkmeister

Differential Revision: https://reviews.bitcoinabc.org/D16999