[cashtab] Remove fbt and I18n from app and extension

Summary: Remove fbt and I18n from app and extension.

Test Plan: npm install && npm run test && npm start

Reviewers: #bitcoin_abc, josephroyking

Reviewed By: #bitcoin_abc, josephroyking

Subscribers: josephroyking

Differential Revision: https://reviews.bitcoinabc.org/D9842