'New user' flow for SSOing into a GitLab.com group [UX]

UX for https://gitlab.com/gitlab-org/gitlab-ee/issues/5292

Overview

To support a new user flow after a user successfully SSOs, we need to consider and support a scenario where a user:

1. Does not have a pre-existing user account on the instance, and/or
2. The connected group enforcing SSO is requiring dedicated credentials, and the user must register a user account specifically for that group.

The main requirement here is a new treatment for the sign-in/registration page; see GitHub's reference image below. The user should understand:

• That their SSO attempt was successful (they were authenticated from the connected identity provider)
• Why they're being asked to create an account
• That their new account will be used to access a specific group
• That after registration, they'll be redirected to whatever resource they were requesting.

References

Solution

• Group managed account
  • Linked (after SSO, straight to group-restricted source)
  • Not linked (after SSO, user needs to register)
  • Not linked, signed in (after SSO, user confirms sign out, then creates a new account)
• No group managed account
  • no existing GitLab.com account linked, signed in (user chooses an account to link)
  • no existing GitLab.com account linked, not signed in (the user needs to sign in or register)
  • existing GitLab.com account linked