[Cashtab] Add tx history

Summary:
Add tx history to main wallet screen for the 10 most-recent confirmed transactions.

This ended up being a large diff. Data from available API had to be manipulated in specific ways to be suited for processing tx history. However, it's still only one feature, and a barebones version (does not process token txs, does not process unconfirmed txs, does not update as txs are sent or received -- features will be added in future diffs).

Unit tests and mocks are added for new functions. Although several files are changed, the only change from the user's perspective is seeing transaction history on the main screen.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9328