Support editing and deleting of dismissal comments

What does this MR do?

This change supports the editing and deleting of vulnerability feedback dismissal comments. It also supports adding a comment if one was not supplied during the initial dismissal. For the backend this is all done through the update action of the vulnerability feedback controller.

This change builds on the work in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/11162

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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

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

Second part of backend work for #10364 (closed)