Include "Deleted" message in a discussion/thread if note is deleted

From @at.ramya's suggestion:

I found that when I delete a discussion - only the main comment in the discussion is deleted, and not the replies for that discussion. I think it would be a bit confusing to have the replies without the the main comment in the discussion - Is this behaviour as expected?

This is currently how it works on GitLab.com, but maybe we should add a Deleted note when notes are deleted in a discussion so it's clear that it once existed. Sort of like how Reddit handles deleted comments in a thread.

I'm not sure if it's technically feasible though. @smcgivern If comments are deleted, are we able to reference them, or are they completely gone?

Proposal