Disable loading vulnerability feedback in MR when user is unauthorized

What does this MR do?

Related to #215570 (closed)

This MR introduces small improvement in MR page to prevent unneeded calls to Vulnerability Feedback REST API and 404's when user is unauthorized to read vulnerability feedbacks. This change also fixes problem with presenting the diff in Security Scanning MR widget, previously (because of 404's on vulnerability feedback API) it was not rendered correctly.

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