T1752

In standardizing the wallet loading screeen to lock the entire app UI instead of screen by screen, I neglected the case of a new user with no created wallet.

This user will see a perpetual loading screen since wallet === false and therefore validWallet will always be false.

This diff handles this case in the App.js spinner.

npm start
Load in an incognito window or browser with no cashtab wallet
Observe that the app does load
Create a wallet
Send a tx and verify the app locks UI for loading and then unlocks when utxo set is refreshed