Filter by labels

[wking]

I don't know what the URL for this would look like (maybe just ?labels=foo,bar on top of our existing scheme), but it would be good to have a way to view all open issues (assigned to a specific user, or in specific repo) tagged with a label. And then we'd walk the dependencies of those issue regardless of tags. This lets you address “I want to push design forward on this repository” and similar. The UI would probably be “click a card label to toggle its inclusion in the root-node filter”.

We should probably also flatten GitHub's milestones into labels, but I'll make a separate issue for that.