Stable shipping assets should go to isolated feeds only if they are actually stable

[mmitche]

Today, if a build is marked as stable, we create isolated feeds for the shipping outputs. Then, any shipping output ends up on those feeds. The VMR has ocassional stable assets from some repos. We don't want these always pushed to isolated feeds, since that will make maestro updates via backflow (or forward) messy.

Instead, individual assets that are marked stable should only go to the isolated feed. To do this, we should propagate IsStable to individual assets, rather than on the whole build itself.