[Cashtab] Move defaultSettings out of Ticker.js

Summary:
Similar to D14102, this diff moves the defaultSettings config out of Ticker.js and into it's own config.

Changed the name to cashtabSettings to reflect the fact that these settings can be updated within the Cashtab app, compared to all the other config files.

Test Plan:

• grep -r currency.defaultSettings src/ and ensure no outputs
• npm test
• npm start
• access cashtab from a desktop browser with no existing cashtab cache and create a new wallet
• navigate to Settings and verify that default values for:

Fiat Currency matches fiacCurrency in config
Send Confirmations matches sendModal in config
Hide messages from unknown sender matches hideMessagesFromUnknownSenders in config

• navigate to Home screen and verify the balance eye toggle state (up top) matches balanceVisible in config
• navigate to Send on a mobile (visit this diff on https://defaultsettings.netlify.app) and verify the camera automatically switches on to scan QR codes in line with autoCameraOn in config
• change currency, send confirmation, balance visibility and hide messages from unknown senders settings and ensure they persist after closing and reopening app

Reviewers: bytesofman, Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D14118