Separate Invites into their own controller

What does this MR do and why?

• simplifying logic and making code more maintainable
• cleanup of invite logic from welcome controller will be done separately due to deployment concerns
• see https://gitlab.com/gitlab-org/gitlab/-/work_items/590048

How to set up and validate locally

note basically trying to emulate feature spec steps in ee/spec/features/registrations/saas/invite_flow_spec.rb

1. Setup to simulate SaaS and restart GDK
2. Enable the refactored_invite_welcome feature flag
3. As an already logged in user, invite a user by email to a group or project.
4. Go to http://gdk.test:3000/rails/letter_opener to get the join now link.
5. Open that in an unsigned in browser window
6. Register as a new user.
7. Ensure registration flow goes as expected and no requirement to create a group is in that flow.

MR acceptance checklist

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Related to https://gitlab.com/gitlab-org/gitlab/-/work_items/590048