Hide CI secret variable values on save

What does this MR do?

• Hide CI secret variable values on save

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Make page-loaded and ajax persisted variables a bit more consistent, https://gitlab.com/gitlab-org/gitlab-ce/issues/42929

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes #42929 (closed)