[Cashtab] Prevent creation of unacceptable agora offers

Summary: Bug fix. the UI was not resetting the max on the slider as expected, nor properly validating for unacceptable offer creation.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17457