[CashTab] Adds webpack aliases and refactor imports

Summary: Adds webpack and jest aliases, also refactoring imports to replace multiple instances of "../../../" to more readable ones, such as '@components/'.

Test Plan: npm install, npm run test, run all tests, npm run start to run app.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8609