Provide a build artifact for each Calico release

[tomdee]

As requested in #662 (comment)
and following on from #864 I propose that we do the following

• Update RELEASING.md to include a step for creating the release archive and uploading it to a github release
• Add a link to the release tarball in https://github.com/projectcalico/calico/blob/master/master/releases/index.md - but how to handle older releases that don't have an archive.
• Decide how to handle the chicken and egg with htmlproofer and wanting to create a PR that links to a new release before the release tarball is actually uploaded.
• update releasing.md to tell users to test the archive download once everything is done
• figure out how to only do this for v2.4+ (and not for master)

Also, maybe

• Cleanup/consolidate the release process that's split across two Makefiles at the moment,