Use onboarding_status for registration type concerns

We should start using the values we have placed in registration_type to drive logic and use during existing onboarding.

The idea is that we should properly populate the registration type via #435745 (closed) and then in this issue we'll be concerned with using it.

Possible cases/methods that will then make use of it:

• trial?
• tracking_label
• group_creation_tracking_label
• invite?
• subscription?
  • stretch...ideally, #418820 (closed) may need to be done first.
• removal of passing of trial param or other params to multiple controllers for situations like this

Plan

• Only use the new db status field in after-registration-onboarding areas
  • Implement with Feature Flag to derisk - !148752 (merged)
  • Ensure no database value for registration types is documented in request specs - !150548 (merged)
  • Rollout Feature Flag - #454680
  • Remove Feature Flag - !148879 (merged)