[Cashtab] Add toggle switch to show Agora prices in XEC or fiat

Summary:
T3711

In general, fiat prices can be more "intuitive." However in practice this is not often the case for Agora spot prices, which may be something like $0.000017 USD per token.

Add a toggle switch to make it easy to switch.

The Switch component is updated to typescript while we are at it. Needed to do this to troubleshoot behavior of "hit switch on one component, toggles it on a different one". This ended up being related to Switch needing a unique name. Important React caveat to keep in mind when rendering 100s of components...especially when they all can spend money.

Test Plan:
npm test

fiat

xec

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D17391