[Cashtab] Use only one app.css for webapp and extension

Summary: Extend use of the REACT_APP_BUILD_ENV variable to deprecate maintaining two app.css files.

Test Plan: npm run extension, open extension and confirm the min-width and min-height css props are there

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14977