Automatic release notes out of PR labels

[sandhose]

This is a start to make the release process easier. The current categories are a bit abritrary, will see how it goes.

To see how it would look like, draft a new release, set the tag to v0.13.0, target the quenting/automatic-release-notes branch and click 'generate release notes'

[cloudflare-workers-and-pages]

Deploying matrix-authentication-service-docs with    Cloudflare Pages

Latest commit:	ec66125
Status:	✅  Deploy successful!
Preview URL:	https://fdb8951a.matrix-authentication-service-docs.pages.dev
Branch Preview URL:	https://quenting-automatic-release-n.matrix-authentication-service-docs.pages.dev

View logs

[reivilibre]

SGTM as a first cut. Time will tell if we want any improvements. But I'm not against having the PR titles (presumably) being == the changelog entries (this is what I do in Synapse with a script anyway)