This adds a `Publish` stage to the combined `Jenkinsfile` which when the
`PUBLISH` parameters is true pushes a __draft__ release to GitHub.
The job remembers the last value of `PUBLISH` parameter selected.
The release uses the contents of the `VERSION` file at the root of the
repo as the name for the release, and leaves the contents to be filled
in by whoever will approve the release.
The automation overwrites - or to be exact, deletes and recreates - the
release, so releated builds for the same release will simply re-create
it. All the built artifacts are included in the release.
Signed-off-by: Jakub Sokołowski <jakub@status.im>