[e.cash] Implement google analytics

Summary:
Dropped the ball on this one and did not implement matching the old site. Did not notice until checking today.

Implement.

Test Plan: Land and check that it is being picked up

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18850