Bring changelog/manager from release-tools project

As a part of https://gitlab.com/charts/helm.gitlab.io/issues/289, we need to pull in release-tools' changelog/manager with some modification to remove logic requiring the use of versions.

We'll need to bring in changelog/manager, and the related libraries from release-tools repository, with modifications to several pieces of code in order to handle operating without versions (Alpha, Beta). We should bring the appropriate documentation for that code, but make a distinct note of the changes as compared to the upstream.

This logic needs to be incorporated, and then added to a new CI step, which will be run on only on the master branch.