Prevent access of non-admin users to SCC Registration page and notification

[cnotv]

Summary

Fixes #15510
Fixes #15579

Occurred changes and/or fixed issues

Restricted navigation to admin only and feature flag.

Technical notes summary

• Added FF rancher-scc-registration-extension
• Added extensions unit tests (first project case)
  • Role must be identified by Rancher as admin
  • Feature flag must be passed by Rancher

Areas or cases that should be tested

Areas which could experience regressions

NOTE: If you stop and restart the feature flag in Docker it will make Rancher crash. Use a new instance to test this out if you have a lot of data.

Navigation to the registration page using the UI or both admin and standard user.

Screenshot/Video

Comparison admin to standard

Navigation and notifications are missing if no feature as admin

Checklist

• The PR is linked to an issue and the linked issue has a Milestone, or no issue is needed
• The PR has a Milestone
• The PR template has been filled out
• The PR has been self reviewed
• The PR has a reviewer assigned
• The PR has automated tests or clear instructions for manual tests and the linked issue has appropriate QA labels, or tests are not needed
• The PR has reviewed with UX and tested in light and dark mode, or there are no UX changes
• The PR has been reviewed in terms of Accessibility

[aalves08]

Gave it a test and works as advertised, so LGTM!

[aalves08]

@cnotv created #15645 as a spin-off of your PR 🙏