[e.cash] Fix v2 hot reload

Summary:
Turns out we do need slightly different CSP values for dev mode.
The next.js hot reload functionality needs the looser 'unsafe-eval' to work, but it is not recomended to have that in prod
Adding a conditional csp value to the next.config

Test Plan:
try npm run dev on master and make some changes. It shouldnt hot reload and there will be a console error
then try on this branch and npm run dev should work as expected

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D18174