@-completion doesn't always work

Completion of @ directives doesn't always work in Reviewable; a known case is when a user writes a comment on a PR in a personal repo where they only have pull permissions. The root cause is that the GitHub API doesn't permit fetching a list of relevant users even though they allow autocomplete in their own UI and effectively reveal the same information. They are aware of the discrepancy and intend to correct it in the future, but as usual there's no timeline.