Consider adding PRs to burn-down chart

[PaulMorris]

Paraphrasing what @kfogel wrote: we should consider including a column of linked PR numbers next to the column of issue ticket numbers in the burn-down chart's feature pop-up overlay.

That would entail tracing through issues to get to the PRs (because the PR won't have the Z-REQ-PSM-* label -- at least, we haven't made a practice of doing so, and ideally we wouldn't have to because every PR should know all the issues it's connected to and vice versa).