Add script which uses GitHub's REST API to fetch issues and pull
requests that have changed since the last release, parse out the
changelog entries from merged pull requests, and print a formatted
version of the changelog suitable for copying into the release
description, the CHANGELOG.md file and the Discord announcement.
The script expects Pull Requests to have a `### Changelog` section in
them with a code block containing changelog entries. As an added bonus
the script can parse out `#<number>` references so that you do not need to
write it out in the ugly `[#id](long-url-with-id-repeated)` format.
Updates documentation and pull request template to describe the
changelog format and usage of the formatting script.
Includes improvements from review and a second look.
Arvids Godjuks