[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