When creating a release draft and uploading assets, provision for the case
that a draft release already exists. In that case, instead of creating a
release, the existing release (changelog) is edited and the assets are
deleted before the new built ones are uploaded.

This commit also introduces the `${gh.repo}` property in build.xml,
so that the Github project could be chosen dynamically. Not really needed,
to be honest, but, yeah, whatever.

Set the ignored branches pattern to `release*` for the normal ci workflow.

Add Ant tasks and macros to deploy binaries to GitHub,
using GitHub's releases.

Adds an Awk script to extract GH flavoured markdown release notes
from the release.moxie file.

Adds `ok.sh` to the repository so that it is readily available.
This is a Bourne shell GitHub API client, used to create a release
on GitHub and upload the binaries.

Add a job to run a build with Java 7.
The job currently only runs on Linux.
In order to run on Java 7, an old Ant version is downloaded and
installed. We use moxie+ant for this, so we build with moxie.

Currently that is Ubuntu 18.04 and Windows Server 2019.
https://help.github.com/en/actions/automating-your-workflow-with-github-actions/virtual-environments-for-github-hosted-runners

We cannot build with Ant on GitHub since the provided Ant
version does not run on Java 7.