Unable to assign PRs to commenters

GitHub recently added the ability to [assign issues to a commenter](https://github.blog/changelog/2019-06-25-assign-issues-to-issue-commenters/) who only has pull permissions on a repo. Reviewable isn't yet aware of these semantics and will disallow any `+@username` directives for such users. (I think it may already permit unassigning from such commenters with `-@username`, if they got assigned via GitHub UI.)