CI: Label PRs with B2-breaksapi if substrate changes are labelled B2-breaksapi (#862)

* CI: Split common functions into lib.sh and make...

... alert_pending_release.sh check the substrate changes for various
labels. If labelled, will be included in the changelog, otherwise will
silently be left out.

* replace non-standard /bin/bash with /usr/bin/env bash

* CI: make lib.sh /bin/sh-compatible

* lib.sh: fix newline chars sometimes breaking has_label()

* check_runtime.sh: tag change with 'B2-breaksapi'...

... if any substrate changes are also labelled 'B2-breaksapi'

* Remove debug exits and uncomment labelling

* exit early if we label with breaksapi

* fix publish_draft_release.sh