[Cashtab] Do not show unacceptable offers to buyers, highlight them uniquely to sellers

Summary:
It is possible for agora offers to be "unacceptable", i.e. the terms of the offer are such that the min buy is smaller than the tokens available to buy.

Cashtab has a bug where validation is not correctly catching this, allowing such offers to be created. But even if (er, when) we catch it in Cashtab, anyone could make such an offer, so we need logic to handle them.

Test Plan:
npm test, see screenshot. will patch the UI validation that is making these possible in a subsequent diff. For now this bug is helpful for testing this.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17455