WIP: Prototype/experiment for GroupSAML SSO Enforcement

What does this MR do?

I've been experimenting locally with a prototype for https://gitlab.com/gitlab-org/gitlab-ee/issues/5291

So far it mostly covers "SSO enforcement prevents access to basic group resources" but I'll probably update it as I experiment.

Currently included:

• Group level access enforcement through UI
• Project level access enforcement through UI
• Redirect to sign in page when UI access prevented
• Background SSO session required for non-UI access. This covers enforcement for git access over SSH, but might also work for API access.
• Displays a different error message when Git access requires a new SSO session.

This MR will be closed in favour of new MRs as progress is made.

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• 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?
• Security reports checked/validated by reviewer