Deactivate the visual_review_bot, remove remaining references, migrate to ghost user

The Problem

We removed Visual Review in #387751 (closed). However, there are historical comments on MergeRequests that were authored by the visual review bot. We don't want to maintain that user type any more as it's related to a feature that doesn't exist, but we do want to keep the review history. This is an aesthetically important but non-timing-critical follow-up to #387751 (closed)

A Proposal

When users get deleted, we migrate their comment history to a Ghost User. This seems to be an acceptable solution, but I'm open to others.

Why didn't we do this already?

There were a lot of other removals happening in the two prior MRs:

So it seemed easier to split this out into a separate change

Edited by drew stachon