[Cashtab] Convert OrderBook component to ts as prep for adding websocket support

Summary:
Per title.

We want to add more features to OrderBook -- esp websocket so that lists/buys/sells happen in real time. Should convert to ts before expanding features.

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D17153