[Cashtab] Improve notification styling

Summary:
Upgrade to latest react-toastify. No longer have to import css. Implement custom styles in line with latest redesign.

Implement typescript for TokenIcon so we can get it to work in useWallet.ts with upgraded toastify.

Test Plan:
npm test

deployed to https://cashtab-local-dev.netlify.app/

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17473