DAG: Enable showing all upstream blockers of a specific issue in a subgraph

[str4d]

We are currently using GitHub milestones to show everything blocking a release, but given that we have release-specific issues in the DAG, it would be much more usable to just tag those release issues with a label, and then have all upstream blockers be shown automatically in the boxes.

For sequences of labelled issues, upstream issues that are already in an earlier subgraph should be left out of later subgraphs for simplicity (i.e. the subgraphs are adjacent rather than nested).