Enable showing issues "blocking" labels

[str4d]

Currently, the only issues (and PRs) that show up in the DAG are ones that have blocking/blocked-by edges in ZenHub. We do see standalone floating issues in the DAG, but that's an artifact of graph pruning: all of their inbound edges are to closed or merged issues/PRs, and they have no outbound edges.

It would be useful to also show some truly-standalone issues, but we don't want to show all of them (there are currently 949 open zcash/zcash issues as of writing). Instead, let's add the ability to show issues tagged with particular labels, and have those show as "blocking" the label (so we'd see a bunch of issues pointing to a synthetic node representing the label). That will enable those issues to be exposed in the DAG, and then linked into other parts of the DAG as appropriate (if we discover such links).

[str4d]

In practice, we've created "label-like" issues for this kind of purpose, and it has caused problems with the sizes of the render. So instead I propose that we allow specifying labels to include issues in the DAG, but that they show up like milestones (a box).

• If an issue has both a label and is in a milestone, it shows up in the milestone box if those are being rendered.
• The set of configured labels forms a priority list. If an issue has more than one label, it shows up in the box of the earliest specified label.