[Cashtab] Move google analytics id out of monorepo

Summary: This stuff is public in web pages, but it's still best practice to keep it out of the repo. That way people don't accidentally copy it on other web pages, fork it around, etc.

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D16975