Fix replying to commit comments on merge requests created from forks

What does this MR do?

Allows you to reply to a commit comment on an MR when the MR was created from a fork.

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

This touches quite a lot but I think it's reasonable. The feature spec is intended to cover all the changes outside the build service.

Why was this MR needed?

I guess this hasn't worked for a little while?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/31772.