[Cashtab] Move defaultCashtabCache out of Ticker.js

Summary:
T3261

Toward deprecating Ticker.js, move defaultCashtabCache to its own file

Note: "cashtabCache" and "cashtabSettings" are both somewhat of a mouthful. However, they reference specific objects. "settings" and "cache" are both general terms and we want to avoid ambiguity.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14364