[Cashtab] Show a spinner while parsed txs are loading

Summary: Currently we just see a blank screen while txs are loading. Show a spinner.

Test Plan:
npm test

npm start and pick a wallet that does has not yet migrated to new tx history. see spinner, then see txs load.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D17367