[Cashtab] [wallet mgmt improvements p1] Add UI locking to settings page

Summary:
T2822

The UI of Configure.js should be locked while async operations that impact wallet data are performed. This diff uses the same approach implemented in Send.js and SendToken.js for locking the UI until the utxo set refreshes. In this case, it is applied to lock the UI until savedWallet state variable has updated.

Test Plan:
npm start
Navigate to settings page
Activate a different wallet
In the dev console, note msg savedWallets from useEffect in Configure.js updated, unlocking UI
Test for the same in the extension by building it locally

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D12841