[Cashtab] Lock nav UI while new settings are loading

Summary:
T1701

Because Cashtab screens were developed incrementally, they have distinct loading context. This means that Configure.js will lock only the Configure.js portion of the screen when the app is in a loading state, and not the full UI including the bottom nav menu. This allows a user to navigate away from the Configure.js screen after a settings page and potentially see unloaded settings on another screen. For example, a user could change the currency on the Configure.js screen and then see the wrong fiat balance displayed on the homescreen because the app has not finished loading the new exchange rate.

This diff applies the global loading context to the full app for the Wallet.js and the Configure.js screens. The Send.js and SendToken.js screens use a different loading context and a separate refactor will be required to correct the issue there; a task is open for this.

Test Plan: npm start, navigate to Configure.js, change fiat currency, note that nav menu is locked until currency is loaded.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9869