We already have a loading spinner on the button, and it feels unexpected for the avatar spinner to randomly appear some time after the user presses the button (after the registration server call concludes and the avatar one starts).

Depends on D8150

The whole stack was tested as follows:

1. Try creating an account with the default avatar
2. Try creating an account with an emoji avatar
3. Try creating an account with an image avatar (that needs to be uploaded)
4. Try creating an Ethereum account with an ENS avatar

I made sure of the following:

1. Loading spinner appeared on the button, not on the avatar, and was present throughout
2. After the RegistrationNavigator is dismissed, the avatar that appears for the user's PRIVATE chat in the ChatThreadList is correct
3. Subsequent delete-account-and-then-register-again worked correctly
4. Various error cases are triggered correctly