Resolve "Fix confidentiality note on epic's comment field"

What does this MR do?

Fix confidentiality note on epic's comment field #219242 (closed). Before the fix, the warning displayed "this is a confidential issue" instead of "this is a confidential epic".

• Add epicWarning vue component that displays "This is a confidential epic ...".
• Change ee/app/serializers/epic_entity.rb to also embed the link to the GitLab documentation on the confidential epic feature.

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 #219242 (closed)