WIP Todo for mention should be bumpable

What does this MR do?

We do not update a ToDo when there is another mention in the same issue or MR. We now update the updated_at for an existing todo when there is another mention to gain the ability to sort ToDos by updated_at in our ToDo list.

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

Surely, for now this is work in progress

Why was this MR needed?

https://gitlab.com/gitlab-org/gitlab-ce/issues/23293

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• 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?

https://gitlab.com/gitlab-org/gitlab-ce/issues/23293