Automate backport workflow

[Drumor]

Based on https://github.com/tibdex/backport

By labeling PRs that need to be backported, This should automate the process. For our needs, we have to define a backport inginious-0.X label and set this label on PRs that needs to be backported.

[anthonygego]

I still think it's more convenient to do the backports manually for the moment, for several reasons :

• Bad merges that need to be reverted happen
• Sometimes master and release branches are too divergent and backporting need manual conflict resolution.
• Fixes aren't always pushed via a PR: if the process is different things may be forgotten.
• We may forget to publish the associated minor release.
• Sometimes we realize a posteriori that some features from the master branch can be backported to the release branch. It is simpler to apply a selection of commits in the same order.