Experiment: Add modal with celebration to the end of user signup

What does this MR do and why?

Adds candidate experiment experience at the end of user signup

as per candidate design

• Adds invite members open at the end of the user journey during signup when they land on learn gitlab
• Adds celebratory text and confetti animation.
  • Adds canvas-confetti npm
  • Uses canvas-confetti Basic Cannon options settings:

  {
    particleCount: 100,
    spread: 70,
    origin: { y: 0.6 }
  }

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/339892

Screenshots or screen recordings

celebration	screen recording
	Screen_Recording_2021-11-12_at_10.20.45_AM

How to set up and validate locally

1. yarn install (for confetti npm)
2. Enable the confetti_post_signup

   Feature.enable(:confetti_post_signup)

3. Register as a new user by visiting http://localhost:3000/users/sign_up(or by clicking Register now from http://localhost:3000/users/sign_in)
4. Answer Just me on the http://localhost:3000/users/sign_up/welcome page(no other fields need filled out/changed)
5. Click Continue
6. On the next few screens, fill out new Group and Project names.
7. Wait for the Get Started with GitLab to finish.
8. Click Ok, let's go.
9. Observe invite modal open on next screen.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.