[Cashtab] Improve eslint settings

Summary: Add .eslintignore so that huge dependency folder is not linted. Remove eslint settings from package.json as these are duplicated in .eslintrc.js

Test Plan: eslint . and no output

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15257