"Rebase and merge" button on github

Hi,

Github now has a "rebase and merge" button which does exactly what we have been doing manually so far.

I think we can just use it. Differences compared to our old way are:

- We don't get any opportunity to add the "Reviewed-By" line, but we can use the "Commit" field instead that is shown by "git log --format=full" and also by Github.

- Github now says "Merged" instead of "Closed".

- It's less work.

I have closed two PRs with that button: #5079 and #5084.

I'll update our Workflow document, OK?