Add sticky confidential issue bar

What does this MR do?

Adds a sticky confidential issue bar when viewing confidential issues

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

Shouldn't be

Why was this MR needed?

Deliverable

Screenshots (if relevant)

Sticky confidential issue bar on desktop

Not sticky confidential issue bar on mobile

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/29741