[meta] New releases should post notifications in resolved issues/merged PRs

# 🚀 Feature request
When a new release is published, it includes information in the changelog about which PRs are included and which issues are fixed in the new version. The publishing process should include automated notification hooks so that those following the issues/PRs will see the message and know that it's resolved.

I'm making an issue for this per [this comment](https://github.com/angular/angular-cli/issues/13212#issuecomment-457889697) from @IgorMinar.

Start by tracing the release publishing process that generates changelog entries for included PRs and fixed issues. Determine where notification hooks could be added and what GitHub issue and pull request events they must update; done means a published release automatically notifies followers of the referenced items.

Written by the indexing model from the issue text.