[Cashtab] Disable google analytics in extension using env var

Summary: Google analytics doesn't work in the extension and should be disabled there. (well, there is extension google analytics, but you implement it from the chrome store dev dashboard, you don't use the same implementation as a website).

Test Plan: npm run extension, load extension, check the console, observe no google analytics related error on app loading

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D14959