Prevent users from switching between new user & restore account

Does the following:

• Prevent navigating back to "create username" screen after successfully registering a username. The app will close instead.
• Prevent navigating to "restore account" flow after submitting a username (whether or not it's successful). An error message will be shown instead: "A username has been submitted. Please reinstall to use restore."
• Prevent navigating back to "create username" screen after restore account has been attempted. The app will close instead.

requested review from @ahmedshehata01

assigned to @kam.elixxir

marked this merge request as ready

mentioned in commit b180f072

merged