Limit the number of comments on a noteable

From https://gitlab.com/gitlab-com/infrastructure/issues/4230, we saw that a user accidentally had a bot that added 15,000 comments to a single commit.

This commit page doesn't load anymore, and each new comment causes the NewNoteWorker to take minutes to run.

I think we should have a reasonable limit on the number of comments on a noteable (e.g. commit, issue, merge request, etc.). Thoughts?