[Cashtab] Domain move

Summary:
T1681

This diff will start showing a pop-up message to users who view the app at cashtabapp.com, advising them that the app is moving to Cashtab.com

Users who view the app at cashtab.com will not see the msg.

So, the diff allows use of a single codebase between two domains, while rolling out migration alerts in phases to users still visiting at the old domain.

Before this diff lands, cashtab.com will be pointed to the production server, 51.210.180.194

Test Plan: npm start and verify pop-up appears (because domain is localhost and not cashtab.com)

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D10274