Extracted the logic to pull the latest release notes, out of
`bin/create-release-tag` into `bin/_release.sh` so that it can be reused
in the `release.yml` workflow, which needs to use that inside
`gh_release` when creating the github release in order to have prettier
markup release notes instead of a plaintext message pulled out of the tag
message.
The new extracted function also receives an optional argument with the
name of the file to put the release notes into, because the `body_path`
parameter in `softprops/action-gh-release` doesn't work with dynamic
vars.
Finally, now the `website_publish` job will only launch until the `gh_release`
has succeeded.