DAG: Enable a view with forward-looking "horizons"

[str4d]

This would be the ability to draw "boundaries" through edges in the DAG, to partition future work into manageable regions. This is sort of like what we currently do with milestones, but a) that is going away in ZenHub (so we need to replace it with ZenHub's milestones), and b) horizons need to be flexible enough to not break when we inevitably skip over ostensibly-blocking issues to get things further downstream implemented. (You could argue the DAG should be updated, but in practice we just keep the earlier issues around to reflect the dependency structure.)