Skip onboarding experience for invited users

What does this MR do?

• skip onboarding if the user has memberships already on sign-up(high probability they are signing up from an invite)

Test locally(gdk)

1. Invite someone to project via email.
2. go to http://localhost:3000/rails/letter_opener/ to see that invite email and click join.
3. click register new user and use the email from invite.
4. click through like seen in screen recordings below and validate there is no onboarding step after that.

Screenshots

Before(sends you to onboarding path)

Screen_Recording_2021-03-22_at_12.03.32_PM

After(skips onboarding path)

Screen_Recording_2021-03-22_at_11.52.58_AM

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #325287