Fix confidential note attribute getter

What does this MR do?

Before we implemented confidential attribute on notes we had method confidential? on Note model that checked the value of noteable (issue).

During implementation of confidential notes we combined confidential check of a note and a notable. If either a note or a notable is confidential, the note is marked as confidential.

That is, however, against the purpose of confidential notes. We want to see confidential icon only when note itself is confidential.

I found one place where we want to see note as confidential when issue is confidential but note is not. I therefore introduced a param to the confidential? method.

I am open to renaming that method for that specific case but don't have a strong opinion here.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Issue #217616 (closed)