[Cashtab] Prevent agora buys of offers above spot
Summary:
Current incentive structure is leading scammers to create offers above spot. These are apparently being bought by mistake.
Disable buys above spot in Cashtab. Show an alert explaining.
Test Plan:
npm test
Reviewers: #bitcoin_abc, emack
Reviewed By: #bitcoin_abc, emack
Subscribers: emack
Differential Revision: https://reviews.bitcoinabc.org/D17466