EE Merge from CE-#48640

What does this MR do?

This MR adds the "/pong" quick action to the comment box markdown; for merge requests only.

Are there points in the code the reviewer needs to double check?

Please review the code cleanup related to the original comments. Also, the test cases were meant to include "not_to". The invariant of /pong hinges on the last assignee and the current assignee being different user id's. Therefore, /pong should return a user that is not the current_user.

Why was this MR needed?

To merge the CC version of this feature into EE.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conforms to the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• EE specific content should be in the top level /ee folder
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

GitLab CE #48640: https://gitlab.com/gitlab-org/gitlab-ce/issues/48640