Stable branches

When we'll be about to cut the first Heptapod 0.17 RC release, the 0.16 series that we are now working on will have to become the stable series of Heptapod.

At this point, we'll probably have to cut a stable branch for Omnibus Heptapod, because upstream dependencies are prone to change at each new minor version at least. In that branch, the tag for intermediate versions of the heptapod/partial Docker image should be changed to 0-16-testing, same as we're already doing in Heptapod Docker.

• start the heptapod-stable branch, to support the 0.16 stable series
• 0-16-testing Docker tag
• new CI cache key in heptapod branch
• documentation for releases on the stable branch, including maybe an issue template for next branching?