[Cashtab] Added toggle hide balance switch

Summary: Related to T2729. Added toggle switch in BalanceHeader component which controls toggleHideBalance setting in cashtabSettings object. BalanceHeaderFiat has also been changed to show or hide user balance in fiat using the same switch. Validation tests have been altered to account for the new setting. CashtabSettings is now passed down through the BalanceHeader component.

Test Plan:
cd web/cashtab && npm start
from the home screen, toggle the switch
observe that xec and fiat balances are hidden
navigate to the send screen
observe that the balances are still hidden
from the send screen, toggle the switch
observe that the balances are now showing

check that setting persists across each component that contains the BalanceHeader

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D12374