Add live summary changelog for pre-releases

[krassowski]

Problem

jupyter-releaser already allows to specify if changelog should be since a specific commit or last stable release. However when cutting beta releases we may want to have both: granular changelog for subsequent point beta releases and a summary changelog (since last stable version) to provide an overview for early adapters.

Proposed Solution

There could be a summary changelog placed on first beta push and updated on each subsequent beta release. Maybe it could use a separate html comment tag to indicate where it should be placed.

[krassowski]

I am also a bit lost on how to generate a summary changelog manually for jupyterlab 3.6.0b - all my attempts so far ended up in an empty changelog.

In addition particular I am a bit confused by this behaviour:

input	result

This is probably a me problem but I would have expected it to run on 3.6.x branch with such settings. I assume this because the "Use workflow from" selector does not show relevant branches when running from releaser fork. Does this only work if we run from repo?

[blink1073]

The "generate changelog" workflow is a bit outdated; it doesn't have the "since last stable release" option.
Under the hood it is using https://github.com/executablebooks/github-activity to generate the changelog, which you can use locally as a CLI.