Resolve "Add a button to quickly assign users who have commented on an issue"

What does this MR do?

This feature adds the ability to Assign and Unassign a user from their comment by clicking a button in the More Actions dropdown.

Screenshots

Screenshot of how the button currently looks:

GIF of the old button UI, but interaction still works the same.

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

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

Closes #191455 (closed)