Group SAML SSO when signed out

What

Allows users with a linked Group SAML account to sign in to the GitLab instance with it

Why

Users otherwise need to sign in twice, once to the instance and a second time to update SAML membership.

Screenshots

Existing SSO page

Identity not yet linked

Related

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/6261

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?