Add tools to automate Cookiecutter releases

[cjolowicz]

The intended workflow is as follows:

export GITHUB_TOKEN=<secret>

nox -rs prepare-release
# Edit pull request description on GitHub...
nox -rs publish-release

Releases require some changes to the documentation, and we always hand-edit the draft release before publishing. So we cannot release the Cookiecutter simply by publishing the automated draft release. Instead, the prepare-release session creates a PR with all required changes, and copies the release notes from the draft release into the pull request description. The publish-release session merges the PR, copies the release notes back into the draft release, and publishes the release.

There is a race condition with Release Drafter here, which will also attempt to update the draft release when the PR is merged. Unfortunately, there is no obvious way to skip the workflow when the release branch is merged. Under normal conditions, the Nox session will have published the release before the Release Drafter workflow spins up, so the workflow will simply create a new empty draft release. IIUC setting the release notes and publishing the release happens in a single transaction, so we're not going to publish a draft release without the edits from the pull request. In the unlikely case that the workflow updates the draft release first, the Nox session will overwrite its changes, and the results will be the same.