Add Desktop Github Release Notes Script

[nsakaimbo]

Description

This PR adds a simplified version of the release notes script that was used in the prior, separate wp-desktop repository.

Because we can no longer depend on consistent commit formatting, this script simply aggregates a list of changes since the last stable version under a generic "What's Changed" heading. It won't be as nicely-formatted or organized, but it's something. 🙂

Motivation

Prior experimentation with our release notes confirmed that having a readily-available list of changes with corresponding (hyperlinked) PRs is highly valuable to interested stakeholders.

Manually generating and maintaining this list of changes for each (beta and stable) release is time-consuming and this task can easily get dropped from one release to the next (for example if the developer isn't available or is unaware, or simply forgets).

The goal is not to have a perfectly-edited list of release notes for end-users but to have a reasonable first draft of all relevant changes gathered in one place. This ensures that a release can be generated at any time and internal stakeholders (like Platform) will always be able to inspect the relevant changeset via Github.

This "first draft" can also serve as a baseline for further editing (for example with amended formatting or descriptions). However, the most important value here is ensuring there is always some available documentation of recent changes.

To Test

Check out this branch and execute ./bin/desktop/make-changelog.sh to get an idea of what the generated changelog might look like.

[matticbot]

Test live: https://calypso.live/?branch=desktop-github-release-notes

[matticbot]

This PR does not affect the size of JS and CSS bundles shipped to the user's browser.

Generated by performance advisor bot at iscalypsofastyet.com.

[mokagio]

Awesome!

I run this locally and got

## What's Changed

* Add release-notes script to Circle publish step
* Add Github release notes script
* Add/migrate wp desktop teamcity (#47368)
* Fix/lint md files (#47453)
* Root redirect: check the logged in/out state when handling the / route, not when installing it (#46567)
* [eslint] Fix rules `no-var` and `prefer-const` (#47357)
* desktop: add docs (#45916)
* desktop: fix file permissions in e2e/tests/lib (#47021)
* enhancement: use contextBridge for IPC message passing (#47513)

Which is what I'd expect by reading the code and running the commands myself.

Left a few non-blocking questions.