Domain registration edge case: background user interactions while registration service is in progress

(Followup issue from https://github.com/wordpress-mobile/WordPress-iOS/pull/11613)

Expected behavior

(Not expected, just my suggestion) user cannot navigate away while registration is still in progress with spinner shown, otherwise UI navigation on registration completion might not work as expected.

Actual behavior

Currently, user can tap on "< Back" and "Help" in navigation bar while registration is in running (shown in screenshot). In the two cases that trigger domain registration flow (Domain Credit and Automated Transfer flow with Business accounts), the behavior might be different on registration completion depending on whether user stays on the registration screen.

Steps to reproduce the behavior

• Prerequisites:
  • Domain credit flow: have a blog with any paid WordPress.com plan (e.g. Blogger plan) and its domain credit still available
  • Automated Transfer flow: have a blog with Business plan and its domain credit still available
• Enter a domain registration flow from Domain Credit or Automated Transfer flow with Business accounts
• Pick a *.blog domain (.blog has high purchase volume so that cancellation rate from testing has less effect on production domain service)
• Note that the same domain cannot be purchased by the same user after cancellation, so make sure this testing domain is not what you might want to own with the WP.com account :)
• Fill out registration details (these will be saved for the next time)
• Submit domain registration, and run the next step right away
• In navigation bar, tap "< Back" quickly and dismiss the domain registration modal, or tap "Help" to bring up another modal
• [Important] If this domain purchase is only for testing, make sure to cancel domain purchase shortly after testing on web in Account Profile --> Manage Purchases (official FAQ)

Tested on [device], iOS [version], WPiOS [version]

iOS 12.2 simulator

Start by reproducing the Domain Credit and Automated Transfer registration flows on the iOS simulator, then follow what happens after submission when Back or Help is tapped during the spinner. Trace the registration completion and modal-navigation entry points. Done should define and verify consistent behavior when navigation occurs before registration finishes.