Refactor business logic out of the welcome controller and into service layer
Currently the ee/app/controllers/registrations/welcome_controller.rb
contains a lot of business logic still, which isn't ideal and makes it hard to reason about the controller layer.
Things like:
- iterable initiation/execution
successful_update_hooks
parsed_opt_in
track_joining_a_project_event
etc - can all be things added into the service layer in ::Users::SignupService
perhaps or in a renamed-under-onboarding version of that service.
There are a lot of things being moved around in &11768, so I've made it blocked by #453979 (closed) for now.