Add alls-green action as an attempt to replace branch protection rules

[ticosax]

https://github.com/re-actors/alls-green

[webknjaz]

FYI old PRs won't report the new check name unless you restart them. So once this is in branch protection, I recommend rebasing them all. For more active projects with many PRs, I sometimes recommend merging the new check, then waiting for some while and then enabling later so most PRs would just get that check reported. Otherwise, they'll remain blocked because they didn't report a new check that branch protection requires.