Prevent author from being an approver

@dbalexandre can you review this please? Background: https://gitlab.com/gitlab-org/gitlab-ee/issues/388#note_12743940

I'd like to apply commit https://gitlab.com/gitlab-org/gitlab-ee/commit/3758f751906ad76a47a40c653c1b07c767772260 to CE, as it doesn't need to be EE-specific. I'd also really like to not have to do https://gitlab.com/gitlab-org/gitlab-ee/commit/ee751a5963937c25ba456311d26bf4103946ac5f but that comment requires it 😕 

See merge request !560