Add editors without GitLab accounts

[jimafisk]

If you don't want a one-to-one mapping of site editors to GitLab users or you want site editors that don't have full access to the repo, we could potentially leverage Netlify's open source git-gateway project: https://www.netlifycms.org/docs/git-gateway-backend/.

This is low priority since this requires hosting and we'd need to charge a service fee to make it sustainable.