This is a follow up issue for https://gitlab.com/gitlab-org/gitlab/-/merge_requests/38571 where it was suggested that perhaps we don't actually need to add `user` to the `context` in use of the ReferenceFilter. The conversation ultimately began around this addition: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/38571/diffs#ec468f9bd8bdff2821bee8cb866c7d1425f7d1da_79_79 The MR adds the concept of a `user` in the reference `context`, which is the "parent" user of a resource that has a GFM reference being added to it. Example context `user`: the author of a ProjectSnippet which is being commented on, with a link to an issue. Previous to this change, the `AbstractReferenceFilter#call` would return if there was no `project` or `group` "parent', but now will also continue if there is a `user` parent.