New CR workflow with GH issues: fetch ALL open issues

[tripu]

https://github.com/w3c/echidna/blob/fb5a7c449e8eb5daf2f8e903a2342155bbc8229f/lib/transition-checker.js#L40

Do proper pagination.

cf https://github.com/w3c/echidna/pull/575

[plehegar]

I wouldn't expect the w3c/transitions to have over a 100 open issues at once. We don't accumulate more than 6/7 transition requests at a time. And for issues related to the transition requirements, we shouldn't accumulate more than 50 open issues at any time.