Add workflow to auto-merge approved PRs

[troglobit]

GitHub branch and pull-request policies support automatically merging approved PRs when tests pass. This is great but we want the added extra twist of a merge commit even after rebase (for tracking).

Since GitHub does not support this policy, the way to do it is via a workflow that rebase and perform a --no-ff merge using the gh tool when all tests pass and all reviewers have approved the PR.