[Cashtab] Code splitting only in prod

Summary:
npm start was broken by code splitting implementation. npm run build works fine.

We do not really care about code splitting in dev. Certainly not to the point where we are willing to break npm start

So, only code split in prod.

Test Plan: npm start works instead of infinite hang with no error msg

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D18288