Enable `update-pr-from-base-branch` on forks #2168

GitHub recently added a new API endpoint for this specific operation and it works across forks: https://developer.github.com/v3/pulls/#update-a-pull-request-branch

It would replace this:

https://github.com/sindresorhus/refined-github/blob/b3e38ad58c87fd1ab927e27368cf3c836aa29041/source/features/update-pr-from-base-branch.tsx#L19-L29

They don't have an API to verify whether the PR is up to date, so I sent them feedback to ask for one. For now, the API we're using for this should work across forks already.