[Cashtab] Allow typed input for agora quantity selection

Summary:
OrderBook was originally built with a slider that only allowed selection of the discrete values allowed by Agora. This ended up being bad UX. Almost impossible to select an amount even close to what you want.

Replace with typed input. The tradeoff is that we must validate this input and also present the user the delta (if any) between what was input and what is the closest thing we can use.

imo this is better UX though. We had many users complain about the slider.

Test Plan:
npm test

live at https://cashtab-local-dev.netlify.app/

Reviewers: #bitcoin_abc, emack, Fabien

Reviewed By: #bitcoin_abc, emack, Fabien

Subscribers: Fabien, emack

Differential Revision: https://reviews.bitcoinabc.org/D17438