CI: Make alert_pending_release.sh filter substrate changes based on github labels... (#850)

* 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