Add Confidential badge to incident sticky header

What does this MR do and why?

This MR adds the Confidential label to the sticky heading for incidents. Before this change, the confidential label was present on incident headings, issue headings, issue sticky headings, but NOT incident sticky headings. By adding this change we standardise this behaviour.

This MR passes an isConfidential property when we render IssueApp via initIncidentApp. This isConfidential property is already setup to correctly render the Confidential label. This was already correctly being passsed when rendering IssueApp via initIssueApp.

Screenshots or screen recordings

Before

before

After

after

How to set up and validate locally

1. In the left hand panel select Issues
2. Click the "New Issue" button
3. Fill in the Issue form, selecting "Incident" for the Type, and checking the "This issue is confidential..." checkbox.
4. View your newly created incident
5. Confirm that the "confidential" label is present in the header at the top of the incident.
6. Scroll the bottom of your incident - a sticky header should appear
7. Assert that the "confidential" label is also present on the sticky header.

To ensure no unexpected side effects:

1. Repeat this process selecting "Issue" for the Type.
2. Assert that the "confidential" label is present on the regular and sticky header.
3. Repeat this process without checking the "This issue is confidential..." checkbox.
4. Assert that the "confidential" label is not present on either heading.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #373884 (closed)