Add confidential status support for new comments

What does this MR do?

Related to #207471 (closed)

• Adds a checkbox to the notes form mapped to the confidential property: The backend for this feature is related to #207473 (closed). The UI related to this is under a feature flag: :confidential_notes.
• Adds an indicator icon for confidential notes.

Screenshots

Regular and confidential comments, and form with confidentiality toggle below (the slashed eye icon has the same tooltip as the help icon)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides

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