Make ACTIVITY.action_archetype_id optional
In practice we make very little use of this attribute when building archetypes and at least in my current projects, I always just add a dummy regex into this field to get around the mandatory requirement. I suspect that is pretty common practice.
I don’t mind this change but technically, 1.1.0 is not the right place to do it because it could be considered a breaking change: some software could rely on having a value there. If we all agree (and I would), we can make this change anyway, though.