Group SAML SSO Page (But not the parts to make the sign in button work)

What

Adds a Single Sign In page to be used by per-group SAML on GitLab.com

This MR does not include the related Omniauth Provider or callback flow, so the button won't actually go anywhere. There is already a Settings page which links to where this SSO page will be.

Extracted from https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4076

Why

This page will allow users who are signed in to join a group, or re-authorize with that group once their authorization has expired.

Screenshots

Mobile	Desktop

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
• EE specific content should be in the top level /ee folder
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?