[CI] Add criticality of release to release notes

[s3krit]

• Enforces presence of C-labels to designate the importance of a release
• Iterates over them on a release, takes the highest present and includes it in the release notes
• Lists the change(s) that caused the release to be that priority

Devops ticket here: https://github.com/paritytech/devops/issues/569

[coriolinus]

If we mandate a C label, then I'd suggest a C0-irrelevant or similar label for PRs which don't affect end-users at all, such as this one, or #1257.

[s3krit]

If we mandate a C label, then I'd suggest a C0-irrelevant or similar label for PRs which don't affect end-users at all, such as this one, or #1257.

Think of the label as something that changes the importance of the user updating in a timely manner - the default importance for the user updating is Low. The importance of a release should never be 'irrelevant' (else why make a release at all). We could also have it so that PRs are by default labelled as low importance, but having it as a manual step causes the author of the PR to think a little about the importance of their PR.

[gavofyork]

Think of the label as something that changes the importance of the user updating in a timely manner - the default importance for the user updating is Low. The importance of a release should never be 'irrelevant' (else why make a release at all). We could also have it so that PRs are by default labelled as low importance, but having it as a manual step causes the author of the PR to think a little about the importance of their PR.

agree on all points.

[s3krit]

There can be some confusing bash in here due to the use of associative arrays, but I'm happy to walk any reviewers through it. In the very near future I hope to re-engineer this script in a more suitable language.

[gabreal]

lgtm

[gabreal]

don't forget